Someone enlighten me. What's the difference between IDA and IDF weber carbs?

IDA's are for full out race, they are known to be on/off kind of carbs, they dont really have much of a progression.

IDF's are for street and are basically the opposite.

IDA's are an older design, they have very little in the way of a progression circuit, and as has already been mentioned, are very on / off sort of carbs.

IDF's are a newer design, which can still be bought new. They were used as standard equipment on some alfa's and fiats. As such they have a better progression circuit.

They both have different flange / stud pattern also.