what is difference between a HIgh Commission and Embassy?

Answer 1

Both refer to an institution/office that represent a country in a foreign land.

The term 'High Commision' is generally used for countries that are a part of the Commonwealth, while 'Embassy' is used by others. Hence, Canada, a commonwealth country has a High Commision in the United States while the US, a non-commonwealth nation, has Embassies around the world.

Answer 2

Julian is half right.

Commonwealth countries only have High Commissions in other Commonwealth countries. So Canada has an Embassy in Washington DC, but a High Commission in London.