UNIT are part and parcel with the UNSC as humanity's defenders against aliens. UNIT is an anagram for United Nations Intelligance Taskforce. The UNIT command take their scientific advice from The Doctor, a renegade Time Lord who is affiliated with humanity.
UNIT has fought many alien races as they try to invade Earth. Some of these include:
UNIT also takes responsibility for the security of Human Colonies, but it's influence thins the further out from Earth you go.