Sans serif fonts were championed by the Bauhaus art movement, who among other things said that design should be functional, and all those little serifs were completely unnecessary. (They also eschewed capital letters. This was more of a statement in their native German with its traditional Fraktur types and Capitalization of all Nouns.)
Sans-serif in typography is synonymous with
gothic (which is not the same as blackletter) and with grotesque.
I think grotesque is as good a description as any. I really can't stand most sans-serif fonts. The most popular sans-serif, Helvetica (named after Switzerland) is almost universally hated. I do like Verdana, though. (It's in the e2 Jukka theme!)
The "sans" in 'sans serif' comes from French meaning "without".
Specifying "sans-serif" in a CSS stylesheet indicates the UA should use whatever font it likes so long as it ain't got them little doohickeys on the ends.