When to use "throw" and when to use "toss"?

Toss has a more casual, lighter, connotation than throw which is a more powerful and deliberate action.

One tosses a towel to one's club mate in the dressing room, but out on the field of play one throws the ball hard in from the boundary.

Toss to me always suggests an underarm movement, whilst throw involves the full overarm action.

This is only an approximate answer to the question and there are no hard and fast rules. You may well hear usages which do not conform exactly to what I have said.

Toss also has some sexual and slang connotations but no point in going into those here.

You can toss someone a rock (so that person can examine it, for instance), but you throw a rock at someone to hurt them.

toss: to throw something, especially something light, with a quick gentle movement of your hand

In addition, toss often implies an upward motion:

toss: to throw upward

(AHD)