Is the following relation a function?
x y
6 > -1
-1 > 2
4 > 3
0 > 3

yes or no (both 4 and 0 are pointing to 3)

Yes, it is a function.
One-to-one mappings and many-to-one can both be classified as functions;
One-to-many mappings are the only kind that can't be classified as functions.