Classes are divided up by a combination of age, weight, and engine package. Most of this is denoted by the class name, for example, “Komet Senior Heavy”. This class uses the Komet engine, is a senior class so it’s for ages 15+, and is “Heavy” meaning the weight is higher. This is a class that would cater more to heavier drivers who are at least 15 years old. There usually is a “Komet Senior Light” class to go along with this, which would be the same engine and age, butwith a lighter weight for smaller drivers.
Karting classes vary a lot from region to region. You might find one engine package is much more popular in your area than the others and if you were to travel out of state, it might be the opposite. As Freddy mentioned, going to your local track and talking to a local kart shop would be your best bet to get information regarding classes and kart packages for where you live.
On a national level, the popular classes in the several different touring series use a variety of engines.
United States Pro Kart Series (midwest and eastern based national series) is a more traditional class structure, using the Yamaha KT100 engine for several classes, but also using TaG engines like the IAME Parilla Leopard.
SuperKarts USA’s national Pro Tour and regional ProKart Challenge (west coast national and regional series) race the Stock Moto package for juniors through seniors and also offer TaG classes that use the IAME Parilla Leopard and IAME X30. Although in 2016 they are going to be only allowing the X30. SKUSA’s class structure is a ladder system for Stock Moto basically. You start as a junior in the S5 class, then when you’re old enough you graduate to the S2 class which is their “semi-pro” division. After that you can run the S1 class, or the “pro division”.
The Rotax Max Challenge series use only the Rotax built Micro, Mini, Junior, Senior Max engines, along with the Rotax shifter, DD2.
If your goal is to someday race nationally, it might be worth keeping some of these series in the back of your head so you can figure out what engines you should be running that will be legal where you race in the future. That way you aren’t jumping from engine to engine depending on where you run.
Of course, the classes offered at local club-level racing are a bit of a free-for-all so that’s why it’s best to see what’s popular in your area.