Few is opposed to many. It has a negative force, meaning almost none:
I have few (=practically no) friends here. Few men are free from faults.
A few is opposed to none, and means some :
I have a few friends here.
Give me a few more biscuits.
The few has a double force, one negative and the other positive. It means, not man, but all that there are :
The few friends I had, left me' means.
I had not many friends, but I had Some and they all left me.
Not a few = a pretty large number :
He gave me not a few (i.e. many) cakes.