Lots of good info in the BicycleMan link above.
Really basic info: If it has 2 wheels, it's a recumbent bike and if it has 3 wheels, it's a recumbent trike.
I ride 2 recumbent bikes, one a short wheelbase and the other a long wheelbase. Pretty easy to tell which is which.
I also share a recumbent tandem with my stoker/spouse.