Bubble tea is a modern Taiwanese concoction of tea with tapioca. You are given an extra-wide straw to sip your drink so that you can suck up the large tapioca "pearls". The sensation is somewhat bubble-like. There are many flavours you can order besides green tea: peach, cappucino, etc.
In Vancouver, one bubble tea usually costs more than $2. I find this drink quite faddish and not worth buying more than once.