Hi,
T2400 is non inverted and N2400 is inverted. You use one of them when running thru an inverting levelshifter circuit such as the MAX232 and the other when you don't, for example.

OT2400 is just like T2400 but the line isn't driven to the idling state, instead it's left floating (made an input). It can be usefull in multidrop networks.