Although one may argue quite successfully with the definitions that follow, they are provided here as an aid to those who are seeking official recognition of their Metis or Indian ancestry, and are based on the regulations of those authorities who make decisions about that official recognition.
Aboriginal means "of the original people", and therefore includes the descendants of the first, or native, Canadians.
In general terms, Metis are Aboriginal people whose ancestry is "mixed" (initially, most often through the unions of non-Indian men and Indian women) over generations. According to the Metis Nation of Ontario, Metis are those people who have at least one grandparent of Aboriginal ancestry.
Indians are Aboriginal people who may have some non-Indian ancestors (e.g., a non-Indian grandfather) but who are descended primarily from Indians. According to Indian Affairs Canada, status Indians are those who have at least one parent entitled to registration under Section 6(1) of the Indian Act.
Inuit are Aboriginal people formerly called Eskimos, and the regulations which affect their entitlement to registration are administered by the Inuit Tapirisat.