Well, idioms are non-literal expressions. Colloquialisms are expressions in casual or informal language. For example, "ain't" is a colloquialism, but it always has a literal meaning so it can't be an idiom. For another example, we might use "yakka" in colloquial Australian English to mean "work". Again, it's a literal meaning, and actually a loanword from an indigenous language (Turrubal - now extinct).

You can do a search for "English idioms" or "English colloquialisms" for plenty of examples.