Model trains and railroads is a big little world, where there is a lot of technology involved in these modern times. There are many concepts and things to know, here are a few of them:
Scale
H0 - 1:87 - The most common scale used. A large railroad usually fills an entire room.
N - 1:160 - A smaller scale where details can be hard to see, but room for a larger railroad on the same space.
There are many other scales, but H0 and N are the most common.
AC/DC
Not relevant anymore. Everything runs DCC or a similar digital protocol, making the tracks act like a combined network cable and power supply. Each locomotive have a decoder with a programmable address, and each locomotive can be individually controlled on the same track.
Blocks
In the analog world, a railroad could be divided into blocks, where power could be switched off a block to allow one train to wait for another to pass. Today this is not necessary, all trains are individually controlled via onboard decoder, and a large railroad is typically operated from a computer with a user interface instead of a controller with a knob.
2-rail / 3-rail
Some systems (Märklin H0) use a middle rail, where the two tracks are connected to ground and the signal is on the middle rail. This allows some loops which would create a shortcut in a 2-rail system, where the signal is on one of the rails and ground is on the other.
NMRA / DCC
National Model Railroad Association (NMRA) have published the Digital Command Control (DCC) specification, which describes all details about the signal on the tracks.