What is the difference between == and =:= in Erlang when used with terms in general?

The biggest advantage of =:= is it returns true only for same terms in the same way as pattern matching. So you can be sure they are same. 1 and 1 are same terms and 1 with 1.0 are not. That's it. If you write function like foo(A, B) when A =:= B -> A. and bar(A, B) when A =:= B -> B. they will behave same. If you use == it will not be same functions. It simply prevents surprise. For example, if you make some key/value storage it would not be right if you store value with key 1 and then get this value if ask for key 1.0. And yes, there is a little bit performance penalty with == but least astonishment is far more important. Just use =:= and =/= when it is your intent to compare same terms. Use == and /= only if it is your intent to compare numbers.