"World Beat" is almost more difficult to define than "World Music", and is perhaps best described in examples. When Paul Simon recorded with Ladysmith Black Mambazo, that was World Beat. When Manu Chao (born in Spain, raised in France) sings songs in 7 languages, tossing in everything from to punk-rock to reggae to salsa to Paris cafe music, that is World Beat. When Cajun band Beausoleil heavily infiltrate their music with Calypso beats and Caribbean instruments, that is World Beat.
Basically, World Beat is just a catch-all term (much as "World Music" is), and although many purists turn their noses up at it, it still gets used an awful lot, particularly in music bins in record stores. Therefore, it's best to have a general idea of what it means.