Colloquial language is casual and conversational: it's the difference between "What are you going to do?" and "Whatchagonnado?"

The word colloquial comes from the Latin word colloquium, which means "speaking together." The roots are the prefix com-, which means "together," and the suffix -loqu, which means "speak." Some may think that colloquial language is not good, when in fact it may just not be appropriate for the context. While it is OK to be colloquial and chatty with friends, it is not acceptable to be colloquial in an essay for school or work.

Definitions of colloquial

adj characteristic of informal spoken language or conversation

“wrote her letters in a colloquial style”
used of spoken and written language