Bubble tea is either juice-and-tea based or milk-and-flavouring-of-some-sort based, sweetened with a syrup, though you can pretty much get it made however you want of course. The "bubbles" are tapioca pearls - you take tapioca balls and boil them for 40 minutes or so, then rinse them to remove some of the starchiness, let them cool, add sugar syrup or honey... and they end up glossy and sticky and chewy. They get sucked up through the oversized straw and then you chew them.
I didn't care for it at first but love it now. And yep, I used to work at a bubble tea cafe and make it.