It's a community of web application security professionals of varying sorts. It's a very weird organization.

OWASP is fantastic organisation of Web Application Professionals. Some of their early work was the publication of the OWASP Top Ten, which went a long way to highlighting the importance web application security to organisations.

But they have become less and less relevant in recent years, rather than more relevant. Their previous work on the OWASP Testing Guide, and OWASP Developer Guide are excellent, but are becoming dated now; there are several efforts to bring this up to date.

The Open Web App Security Project has a pretty good (but alas not very up-to-date) catalogue of common web application exploits.