Slot cars are a kind of toy. Being car oriented, usually played with by boys.
They basically consist of a track with a slot in it (hence "slot car"). To be consistent with the other part of the name, there are little cars involved too. Slot cars are electric toys, usually plugged into a wall socket, though I have seen battery operated versions as well.
There are metal ribbons within the track, on either side of the slot. This is where the electricity flows. It's DC current flowing through there, at a reasonably safe voltage, so though you can cause sparks and such, you don't have to worry about your child being electrocuted.
Another part that's involved is the controller, usually a pistol shaped affair, with a moveable trigger.
The toy functions by putting the car on the track. Electricity makes the car move when you apply the current with the pistol-shaped trigger. Though this sounds fun on it's own, the fun is enhanced by the fact that a modicum of skill is involved, as there is only one pin in the front part of the car holding the car on the track. The tires on the car supplies additional traction. If the car enters a curve at too high of a speed, the momentum will overcome the traction provided by the rear tire and/or pin in the front, resulting in a spinout or, in more severe cases, resuting in the car flying off the track and into your parent's house's wall.
There are various sizes of slot cars, the most popular is the HO scale, which is identical to the HO scale in the model train hobby. Other popular sizes include the much larger 1/32nd scale size. Many life-like and not-so-life-like cars are available, ranging from ugly Ford Thunderbirds from the 1980's to Formula 1 race cars.
Popular brands:
info from www.hoslotcarracing.com and outta my head.