From publisher blurb:
The summoner is a sub-class of magic-user that gains his magical powers by making a pact with an outsider, a being from the outer planes. This pact is one of mutual benefit, as both the summoner and the outsider, known as an eidolon, gain in power.
This eidolon physically manifests on the material plane with the summoner. Its physical shape is determined when it is first summoned. It will perform tasks for the summoner, including fighting as well as casting spells.
Although primarily intended for first advanced edition, it also contains tables for other popular versions: original edition, basic and expert edition, and the cyclopedia edition.