Minecarts are a vanilla item that is used to ride down Tracks. They are also used to craft a few different types of carts from the Railcraft mod. There are several types of minecarts in Railcraft: TNT carts, Tank Carts and others.
In Railcraft, the functionality of minecarts is changed minutely. Minecarts are now weighted, and will not move as easilly. Also, minecarts are now a physics object, meaning that they now are solid as opposed to passthrough-able.