Essentially the glass bedding process builds a perfect "hand in glove" fit for your action to your rifle stock. Done properly, it will allow your action to sit in a correctly nestled "bed" and free float your barrel, which leads to greater accuracy.
I am running short on time at the moment, but I will fill in the "How To" later if one of the guys doesn't beat me to it....
EDIT: Okay, so here is a basic How To on glass bedding. First you need a glass bedding kit, they sell them like this one though depending on the size of the firearm, you might need a little more. It's also a good idea to have two sections of rubber hose / surgical tubing about 12" to 14" long. You will also need an Exacto knife, or razor knife, for the final product.
Disassemble the stock from the action.
Clean the stock with acetone or other type of cleaner and masking tape off the areas outside of the action contact points that you don't want the bedding compound to run over into. Generally just outside the front of the action ( like the recoil lug if your weapon has one or right where the barrel connects if not ).
You also want to tape off the area around the bolt handle, the trigger/trigger guard, etc. make sure and tape up over the rails on both sides of the stock as this stuff will squish everywhere once you make the "mold".
DO NOT TAPE OFF WHERE THE ACTION ACTUALLY SITS. You want the compound to stick that surface, the tape is to keep the compound off the rest of the stock.
Clean the action with acetone, or other cleaner.
Mix up the bedding compound per the directions.
Using a spatula, or popsickle stick, or whatever you have available, you are going to pour/paint the compound all around the inside of the stock where the action sits. Brush it around liberally as having too much isn't a problem, but going back in to do fill in work is a pain in the ***.
Once you have a good fill in the action seat, press the rifle action into the compound and press it in tight. You want that bedding compound to squish and run so that it covers everything.
Wrap the surgical tube around the action, and the action only - NOT THE BARREL - to hold it to the stock just as you would have it held in place by the action screws, and tie it off.
Using a marker, write the time on the extra tape of when the compound will be almost set up, but not completely hardened. All compounds are different, so you might want to experiment first. You want it tacky, kind of solid, but not runny and not stiff as a board.
When it reaches that consistency, unwrap the tubing and pull the action. If it was cleaned properly, and the bedding compound is like modeling clay type of consistency, the action should pop right out and leave you a nice impression of where it contacts the stock at.
Using the razor knife, carefully trim away everything that isn't part of that impression, and trim away everything that is at the edge of taped off area.
Clean out the action screw holes and trim all the area around the trigger/trigger guard/bolt handle. etc.
Remember, you are looking for an exact impression of the rifle action itself.
Remove all the tape.
Let the unit sit over night and become completely hardened.
In the morning, if all went well, when you put the action back in, and set the action screws, your action will now be glass bedded, your barrel will be free floated and should shoot better.
It's not a hard process, but the first couple of times you do it, you are probably going to be using some new swear words.
I enjoy doing it - but a lot people do one and never want to do another...LOL
Semi auto or bolt rifle with a magazine is still the same thing. You just have to mask off the entire inlet for the mag well and you are good to go. Cut around the area as you would anywhere else you don't want the bedding compound to be permanent and let stand. Reassemble and try the magazine feed. If it's a little tight, use the Exacto/Razor Blade or you can use a Dremel....
I pretty much agree with what dillinger said (as usual),but with a few minor changes. What your goal with glass bedding is the skin tight fit between action and barrel to eliminate any and all movement between them. That also insures you may remove said rifle from stock and reassemble without shift in zero,or p.o.i.
Personally, I glass bed a LOT of rifles so I save time and material by using modeler's clay instead of tape or hose to build dams. The type of bedding I use is "acraglas gel". This gel end cures in same amount of time, while being MUCH easier to work with as it don't run/leak all over the place. They also offer "steelbed" which is same kit but with very small steel particules. http://www.brownells.com/aspx/ns/store/ProductDetail.aspx?p=1038&title=ACRAGLAS GEL~ It basically comes with everything you need however I highly prefer to use this release agent http://www.brownells.com/aspx/ns/store/ProductDetail.aspx?p=1045&title=ACRA-RELEASE? With this release agent, all you have to do is liberally spray the cleaned metal and you're done (with release agent). In preparing the action area to be bedded, I use a bit in dremel tool to slightly deepen area and leave rough texture in order to give the bedding more secure bonding strength. DO NOT FORGET to use release agent on ALL metal surfaces including action screws,trigger housing,and all other exposed metal-EVEN if you THINK it'll never reach there. That's another advantage of using modeling clay-all you have to do is form it into any crevices the bedding MAY flow into,clean it off and reuse next time. Only by doing it will you learn exactly how much bedding to mix up/use. Oh yeah-did I mention to BE SURE and use release agent/clay dam to ALL parts it can possibly be exposed to bedding compound?