Operant conditioning is the process by which an organism learns by consequences, in that behaviour is either reinforced and more likely to be repeated, or punished and the likelihood of repitition decreased.
Classical conditioning is the process whereby a reflexive response is triggered by a previously neutral stimulus due to continuous pairings between a stimulus evoking an innate response (UCS) and a neutral stimulus (becomes the CS).
Definitions might be a little lacking, my brain is running quite well at the moment (finishing an English piece at 1am; brilliant)
In contrasting the two, I would say the role of the learner is different as in classical conditioning the learner is passive and in operant the learner is active as they must first exhibit behaviour before consequences can follow. Classical involves reflexive behaviour whereas operant is more complex. Acquisition, recovery, discrimination, extinction and generalisation are all similiarities... I think. Hm... I'll come back to this thread in the morning and see if I can better answer the question -.-"