As mentioned above, "code" refers to the height of rail in thousandths of an inch: Code 100 is .100" high. Other common "codes" are Code 83, 75, 70, 55 and 40. The first three are seen in finer-gauge HO, the latter two in N. Code 100 is the most common for HO, but is actually quite a bit too big, scaling out to be a bit heavier than the heaviest rail used today. Code 83 is a good "happy medium", it's a little smaller but still pretty easy to work with, although equipment with deep flanges (European prototypes or old toy-train stuff) may have trouble with it. Code 70 is used for fine-scale "Proto:87" model railroading, which requires mostly handlaying track, different wheels with prototype-thickness tires and flanges, and a lot more detail--NOT something for the beginner.
O-gauge three-rail doesn't really fit into the "code" world, but two-rail O scalers use Code 148, I think, and fine-scale O scalers (Proto:48) use, I think, Code 100 (which is of course about half as big in O as it is in HO.)
Code 100 works fine, though, and is cheap, and Code 83 is getting more common and therefore cheaper. They'll do the job.
***DO NOT USE YOUR LIONEL TRANSFORMER FOR HO TRAINS!!!***
It runs on AC. HO scale trains run on DC. YOU WILL DESTROY THE ENGINE and possibly cause fire or injury.
I repeat:
***DO NOT USE YOUR LIONEL TRANSFORMER FOR HO TRAINS!!!***