Speed controllers
There are two types of speed controllers.
1) Mechanical Speed Controller, MSC - This type of a controller is basically a big variable resistor, controlled by a servo. It can normally achieve three different speeds (three-steps), and does no have a reverse option. Advantages: there is only one that I am aware of: cost!
2) Electronic Speed Controller, ESC - This type of a controller is a mysterious black box (to the end user). This means that it has a lot of electronic components, all of which are not visible to the user. It supplies the motor with pulses of the appropriate voltage. Since it is electronic, and the insides are digital, it has many more steps / speeds (up to 255), which makes the running the car seem much more life-like. These also do come with a reverse option. Advantages: better control of the car; runs much cooler; can handle higher battery cell packs; can handle hotter motors without melting. Most drivers today run with electronic controllers. It is actually quite rare to see a car with a mechanical one today.
Most manufacturers will have an explanation of all the terms that go along with the ESC; unfortunately each manufacturer uses a different term to name the same thing, and further, each manufacturer will stress a different thing on their product (it's called 'marketing'). The things that you should concentrate on to start of with: reverse (this is a yes / no type of thing - for off-road go with a 'yes', for on-road you could go either way), number of cells (this is generally a range; you will want one that can definitely handle 6 cells, which is almost all ESCs on the market today), motor limit (the number of turns on the motor that an ESC can handle - remember: the lower the turns = the hotter the motor = get better ESC so as not to damage it).
