'swimming' can be both a noun and an adjective. 'swim' can be both an adjective and a verb (and occasionally a noun). So these are correct: -swimming trunks or swim trunks -swimming suit or swimsuit (one word) -swimming classes or swim classes -swimming costume (although this term is not really used in American English) -swimming programs or swim programs 'Swimming' is an adjective in each case meaning "related to the act of swimming". "Swimming is a valuable skill." is correct. (In this case, "Swimming" is a noun meaning "the general act or sport of swimming".) "Swim is a valuable skill" is incorrect.

All of these things are interchangeable. Personally I think it sounds much better to say "swimming lessons," "swimming trunks," etc rather than "swim."

Swimming trunks are for men and a swimsuit is for women. Swimming costume can be used for men and women. They are what you wear when you are swimming. A swim class is when you learn to swim with a swimming teacher. In part 2 swimming is used because the people are doing it. They are swimming. It maybe in the swimming class or that swimming is a valuable thing to learn. I was a beach Surf Lifesaver and swimming teacher in Australia before i moved to live in Asia. I had to save many people who could not swim. This is why we never say we play swimming. It can be dangerous if you do not know how to do it. We go swimming because it is an important skill. It is lots of fun to if you are good at it.