This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. A manager keeps your name in the loop. He guides your career and helps
you make choices that are difficult for a performer to make because he/she
can't step back and look at the whole picture. He keeps your name in the
mix, so when something comes up people already have you on their mind. An
agent just tries to sell you to anyone. The manager is (or should be) more
selective. If you are looking for a manager, look at his/her clients and
see what they are doing and if they are going to have long term careers,
etc. Alot of the so called " best " managers in comedy today are selling
their clients out so they can get ahead and look good.