Someone else can probably explain this better than I, but... "return" takes a value and puts it in a minimal context that still yields that value. I think of "return" as saying "take this ordinary value and make a monadic value out of it, in the most straightforward way possible". Consider the Maybe monad, for example. What kind of value would you expect the expresion "return 7" to create? Well, there are only two constructors for Maybe values, Just ___ and Nothing. The latter wouldn't do any good, because it doesn't encapsulate the value "7" in any way. So the only choice is "Just 7".
= is like function application, only instead of taking a normal value and feeding it to a normal function, it takes a monadic value and feeds it to a function that takes a normal value but returns a monadic value. It provides a convenient notation for chaining a series of monadic computations together.