A phrasebook is generally a quick and dirty type of foreign language guide. Without dwelling on points of grammar, it teaches basic vocabulary and useful phrases. Phrasebooks will often mangle the accepted romanization of a language in order to convey pronounciation to readers who aren't used to reading alternate pronounciations of their own alphabet. And that's good -- I'm all for anything that'll get people to pronounce word correctly.

Phrasebooks are useful things for the casual traveller, but they'll never tell you the whole story. For example, way back in the day I bought a Japanese phrasebook. Reading it now, five or six year later, I see that it used a very high politeness level that isn't actually encountered in a lot of everyday speech.

So if you're going to a foreign country, and you have the time, take a class. Or at least buy a proper textbook to supplement and augment your phrasebook.