There's different types of drinks you can order at the places here depending on the size of the shop.
The harmless* ones are the fruit smoothies and slushies and I personally think the "milk tea" (sort of like iced black chai, I can't think of a better comparison but meeeeeeh.. it's not one of my favorites, too much and it drys out my tongue, LOL!) that started out the whole fad is an aquired taste. Before Starbucks opened on every other corner in Hong Kong, hot milk tea was the drink of choice for caffiene
The typical tapioca used are dark and round (kind of like lightly sweet, mushy gummy candy), some fancier (usually Japanese) places have different/rainbow colors tapioca balls and Vietmese places have "grass jelly" that are about the same with a stiffer texture, not as chewy and are long strands instead of round. You can also buy tapioca at Asian supermarkets or kits to make your own tea at home.
My staple is a mango shake but I like lychee, taro (root - tis a very pretty purple color and usually just tastes sweet-ish) and almond (w/jelly). I tend to crave the tapioca more than the drink though
*..do NOT ever try Durian fruit if it's on the menu if you are not adventerous, it's quite the "delicacy", just the smell of it makes me run away as fast as possible.