Specific functions of an IT Department

As an IT consultant, I like to think that I bring some of
the functionality of an IT Department to my home
business and small business users. But what
specifically are those functions? My Google searches
have come up largely empty – nobody seems to have
taken the trouble to define “IT Department”.

What would be a list of specific functions of a generic IT
Department at a mid-sized company (e.g. systems
maintenance, security, backup, archiving, etc.)?