There is little if any difference. The meanings overlap. "Postpone" is a much commoner word.

"Postpone" suggests that there was something on a schedule, to be done at a particular time, and it is now going to be rescheduled for a later time. 

"Because a major snowstorm is predicted, we are postponing Barbara's February 9th birthday party until February 16th."

"Today's agenda includes a vote on the construction project." "Mr. Chairman, I move to postpone action on this matter until next week, when the Finance Committee will be ready with final numbers on the expenses."

"Defer" suggests that you are deciding when to do or consider something for the first time. You aren't changing a date or a time that has already been set.

"Should we put an estimate for that building in the budget?" "Let's defer that. We haven't even decided whether it's going to be a yurt or a cabin."