They are used humorously to name like that a man who has the most power and influence and the highest social position in a particular group. I have never heard of the «alpha female», by the way. xD Not many females may be named that way, I guess. :)

Got to Wikipedia for this kind of stuff.

They are not derogatory or offensive.

I have only heard of people using "alpha male" before. I am interested in knowing the answer too. Maybe someone else can explain this question better.