The Minecart with TNT is a type of minecart added in the 1.5 (the Redstone Update) and it's actually a combination of a Minecart and a block of TNT.
The minecart is used to blow up blocks in moderation. When it explodes it will not destroy any rails beneath the minecart and any blocks that the rails are on. They only blow up when an Activator Rail is underneath them thus making your minecart system fail safe, as you can turn activator rails on and off. It also is often banned from servers since it is a good way for griefers to destroy mines or roads with rails.