Black Flag was a punk rock band in the late 70's and early-mid 80's. They have a very heavy grinding sound carried by Greg Ginn on guitar, and angry vocals by a long series of singers who came and went frequently. Later on they took on Henry Rollins as a permanant singer.
They are famous for a few particularly obnoxious songs, mainly "TV Party" (which appears on the soundtrack of Repo Man), a song called "Six Pack", as well as an ever changing and very raw cover of "Louie Louie".
The thing that makes Black Flag particularly interresting in my opinion is that unlike most punk bands, they do not draw so much from heavy metal or pop music as from blues. If you listen to the patterns in their lyrics and to their guitar solos, you can hear that quite clearly.
They are contemporary with Fear and The Circle Jerks.