A company-sponsored event in which cheap beer is served to the employees.
The point behind a beer bash is supposedly to get co-workers to mingle and talk in a social
setting, but usually people just hang out with their friends in their department
and end up talking about work anyhow.
dot.com parties are better because even though you only talk about work, at least
you talk about work with strangers (or, at the very least, potential
employees/employers/investors.)
The beer bash is sort of a watered down imitation of the Japanese
practice of "drinking with your boss", except that at a beer bash, things you
say when drunk will still be held against you.