Colloquialism and Idioms?

I just need to know something for my vocabulary word. Is there a distinction between colloquial and idiomatic expressions? Colloquial expressions are used for a familiar conversation instead of formal speech or writing. Then an idiom is an expression whose meanings can't be inferred from the meanings of the words that make it up BUT are, most of the time, commonly used phrases. Sooooo...are idioms like a colloquial metaphor or vice versa? I'm basically just asking for the similarities and differences between colloquialism and idioms.