There are two kinds - fruit and milk. I don't fancy the milky/tapioca stuff, but the fruit is very refreshing. It's basically a soft drink (eg cherry juice) with bubbles of a contrasting flavour in it. The bubbles remind me of smaller versions of those bath pearls that used to be in fashion when I was young, but they are filled with fruit juice rather than bath oil. You drink it through a wide straw, so the bubbles don't get stuck, and the contrast between the two flavours is lovely.
Don't knock it till you've tried it
. It is widely available in Chinatown areas or near universities.