The stuff that you get at the store to put together pieces of PVC is labeled "Cement." This is a misnomer. The "cement" class of adhesives, such as contact cement, rubber cement, etc, all are used to bond two, usually dissimilar, materials together. PVC cement is actually a solvent. Welding involves melding two pieces of the same material into one. (Hmm, I wonder if there is a connection).
When you solvent weld PVC, you are actually turning the two sides that you are joining into PVC mush, the molecules all blend together, and what you are left with is essentially one single piece of PVC.
Step 1: Materials
-PVC Primer (it's usually purple, some places sell a really nice clear variety, if you want it to look nice you can get this stuff. Some spudders like the industrial look of the purple stains)
*Often you can get dual-packs of PVC cement and primer
PVC primers and solvents contain mostly ketones. Using commutative logic here, ketones are strong solvents of PVC. Aside from bonding applications, PVC should NOT come in contact with ketones, as it stands a chance of weakening the pipe. Ketones, such as Acetone and Methyl Ethyl Ketone, should NEVER be used as fuel for combustion spudguns.