A corner is a place where two sides or edges of something meet. You usually say that something is in a corner.
Put the television set in the corner.
Flowers were growing in one corner of the garden.
When two streets meet, you refer to each of the places where their edges meet as a corner. You use on when you are talking about the corner of a street.
There is a hotel on the corner of Main and Brisbane Streets.
We can't have police officers on every corner.*