meaning - difference between act and deed

I've been searching the internet, but have not quite found a satisfactory explanation between an act and a deed.

Both seem to have kind of a meaning of something done, though through my google and etymology searching the only apparent difference I can find is that "act" slightly hints at something more in the process of being done. (although not necessarily)

Surely there should be more of a difference between the two words. If they mean the same thing surely there would be no need anymore for two words and I also know act and deed can not always be interchanged in sentences, but what is the difference exactly then?

I'm left utterly confused, hope someone can help me and explain the difference...