A 15 amp switch can be used on a 20 amp circuit as long as the load it is controlling is less than 15 amps.
15 amp receptacles can be used as long as there are more than one receptacle on the circuit. Bear in mind a duplex receptacle is TWO receptacles.
Example: You cannot put a single (simplex) 15 amp receptacle on a 20 amp circuit, but you can put a duplex.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.