The Tok'ra are the spawn of the Goa'uld queen Egeria, who programmed them to have a distinctive relationship with their hosts. Unlike most Goa'uld, the Tok'ra do not seize complete control of their hosts, but rather act as symbiotes, allowing the host considerable autonomy.
Severly limited in numbers, the Tok'ra do not operate in direct combat. The Tok'ra operation works highly covertly, and most of their affairs until recently involved the use of subtle political manipulations to try to have the System Lords wipe themselves out while preventing a single, powerful Goa'uld System Lord from coming to power over all others.
Thanks to genetic memory, the Tok'ra are at the same scientific level as the Goa'uld and have developed devices and weapons specifically for use against Goa'uld, including a particularly potent toxic gas designed to wipe out Goa'uld.