If anything is unclear or needs to be added let me know. I didn't want to include a description of every type of perk, so if you want to see all of them you will have to search around, but these are a few of the basics.
Bongs come in a few shapes, the most common are straight or beaker bottom. Each one has its benefits and neither is technically *better*. Its all about preference here. Straight tubes are known to clear faster generally, while beaker bottoms have more volume and are often said to cool smoke a little better because of the extra volume of water.
There are also bongs either with a perk or without. Perked tubes have a second (and sometimes third and fourth..etc) chamber for filtration. There are many many types of perks. Common forms are Tree perks and Circ/showerhead.
Next there are bongs with stems and stemless bongs. Stemless bongs don't have the traditional downstem going into the water, but instead have a fixed tube coming out of the side. Generally these bongs have a form of perk as their primary filtration method. Bongs with stems have a glass tube that is generally removable that sticks out of the side of the main chamber. One end is in the water, the other end is where the bowl attaches. Downstems come in several varieties, from just a standard one hole to diffused, to shower head. Some even have several trees similar to a tiny tree perk and others are super diffused in a style known as gridding. (more on this later)
Stemless bongs also come in 2 additional classifications. 90 degree or 45 degree. This refers to the angle of the stem that comes out of the bong where your bowl goes. This is important when considering an ashcatcher (more on this later)
The bowls and downstems of bongs come in specific sizes, the most common are 14.5mm (sometimes just called 14mm) and 18.8 (sometimes called 18 or 19mm). A 14.5mm bowl will only fit into a 14.5 mm joint, and the same goes for 18.8mm bowls. Similarly (on tubes with downstems) a 14.5 downstem will only fit into a 14.5 joint on a bong, and 18.8 will only fit into 18.8.
There are several points where joint size is measured, and all of them have to line up. These points are:
1) the opening on the bong itself.
2) the outside size of the downstem joint
3) the inside size of the downstem joint
4) the joint size on the bowl itself
If the bong opening (1) is 14.5 then you need a 14.5 downstem, or if a stemless bong then you need a 14.5 mm bowl. If this is 18.8 then you can get either an 18.8 to 18.8 downstem or a "low profile" 14.5 to 18.8 downstem. In the case of 14.5 to 18.8 the first number refers to the size of the joint into which you put the bowl(3), and 18.8 refers to the joint on the outside of the downstem (2) (which would then go into the opening on the bong.) Finally if you have a 14.5 downstem then you will need a 14.5 bowl, if you have a 14.5 to 18.8 downstem then you need a 14.5 bowl, and if you have an 18.8 to 18.8 downstem (also called just 18.8) then you need an 18.8 bowl.
To further clarify because this often confuses people, here are some example setups using the same numbers above (1 is bong opening..etc)
1,2,3,4 all 18.8
1,2 are 18.8, 3,4 are 14.5 (low profile downstem setup)
1,2,3,4 are 14.5
Any other combinations require some form of converter
14.5 is a smaller size than 18.8 and therefore less air can travel through, and therefore is associated with more "drag".
Drag refers to resistance to airflow..more drag means you have to pull harder to get smoke to travel through the bong, less drag means you have to pull less hard, which will feel more like breathing naturally.
A few other things that can be on a bong are splash guards, ice pinches, and carb holes.
Splash guards are usually a dome or something similar near the top of the tube that has no effect on filtration at all, but only serves to help prevent you from ever getting nasty bong water in your mouth.
Ice pinches are some number, usually 3, of fairly small conical shaped intrusions into the tube of the bong that hold ice should the user decide to put ice into the tube. When you smoke the smoke is pulled through water and then over and around this ice, thus cooling it down.
Carb holes are smallish holes in the bong in the wall of the main chamber. You cover the hole while you fill the tube with smoke and then take your finger off the hole to clear the bong. Most bongs don't have carb holes. Normally you simply pull the bowl out to clear the bong.
Ashcatchers, carbon filters, bowls.
An ashcatcher (A/C) is similar to a perk inside a bong, except it serves to filter the smoke before it ever enters the bong as well as to prevent ash from getting into your bong itself. These get dirty so your bong doesn't have to. It fits inside the joint where the bowl would normally go on either stemless or bongs with stems. These also come in many many forms including tree perks, shower head, inline, and several others.
Ashcatchers come in 2 major styles: 90 degree or 45 degree. This refers to the angle of the tube that connects the ashcatcher to your bong. It is important to match this to your bong. If your bong has a stem then in all likelyhood you will want a 45 degree ashcatcher. If it is stemless then you could need 90 degree or 45 degree. When you put your bong flat on the table if the opening you put the bowl into is pointing straight up then it is 90 degrees and you should have a 90 degree ash catcher as well. If the opening is angled then it is at a 45 degree angle and you will need a 45 degree ashcatcher.
Carbon filters are small bulb shaped sections that go between the bowl and either an ash catcher or the main body of the bong. They are filled with carbon and serve to filter the smoke and keep ash out of your bong.
Bowls are the actual bowl where you put whatever it is you are smoking today. These come in 14.5 and 18.8mm sizes normally. There are several styles of bowls, the most common being:
push style bowls (one hole at the bottom),
pinch bowls (similar to ice pinches, but much smaller scale. The intrusions in this case hold your herb, and you are less likely to pull bits through with this style over a pinch bowl),
and Disk diffused (aka "DD." These are often regarded as the best option, and look similar to a wagon wheel in the bottom of the bowl. There is great airflow and very little herb gets pulled through with this style of bowl.)
Other Terms and Options
1) "Gridding/Gridded" This refers to a method of increasing diffusion by dividing the number of holes through which the smoke filters into several more, smaller holes. This is done by putting a small line of glass across the hole the smoke travels through. For example, if you had a basic tree perk with 1 slit on the side of each arm and 5 arms you would have a total of 5 holes. If those holes each had one gridline then each hole would become 2 holes, leaving you with 10 points of diffusion now. If you have 2 gridlines then each hole would become 3 holes, making 15 points of diffusion, and so on. With each additional gridline you get smaller holes and a greater number of holes, resulting it more, smaller bubbles. This means a cooler, better diffused, smoother hit.
2) "Worked" This refers to colored glass used in the process of making the tube. There are many forms of worked glass ranging from "fuming the glass" which applies color, but isn't a solid line..more like a translucent dye, to solid line work, to inlays, to multi-layered crazyness. Entire threads could be and have been devoted to the discussion and appreciation of this artform, and to learn about it the best thing you can do is talk to a glass blower or just read like crazy online, as well as looking at examples of worked glass.
Any piece of glass *can* be worked. Including your bowl, your downstems, your perks, the bottom of the tube, the sides, the top lip of the tube and everywhere in-between.
In addition to colorwork there is also sandblasting and etching. This involves scraping a design into the glass in some way, which results in a whiteish color and a slightly rough texture.
A few pictures to help out those visual learners. (note all pics are from the Grasscity store.)
1. A standard beaker bottom tube
2. A standard Straight tube
3. A diffused downstem.
4. An "inline" style bong. A form of stemless
5. A showerhead stemless bong *With a Perk* (the chamber about half way up the tube is the perk)
6. an Inline Ashcatcher. Note that this is a 45 degree ashcatcher, as the joint sticking out of the back is at an angle.
7. A worked Ashcatcher. Note this is a 90 degree ashcatcher. The joint points straight down (i know the picture is at a slight angle, but imagine it actually in a bong with the bowl pointing straight up..the joint would then point straight down)
8. an ashcatcher with both an inline and a tree perk.
I couldn't find any examples of Disk diffused bowls on the grasscity store, so your best bet is to search. I didn't want to steal anyone's pics without permission, so if anyone wants to attach examples of anything feel free. For an example of a stemless tube with a 90 degree circ ashcatcher you can check out the "my setup" link in my sig.