Say and tell are basically the same in meaning, but
grammatically they are different. Tell must be followed by a
person as a direct object: to tell someone something. Say does
not have a direct object: to say something (to someone), e.g.
He told me that I should wait (or He told me to wait). He said
that I should wait (or He said to me that I should wait).
Talk about and discuss are basically the same in meaning,
but discuss does not need about or any preposition after it.
Speak and talk are basically the same in meaning and
grammatically – to speak or talk to someone about
something is also the same.