jūng gwok chàh
zung1 gwok3 caa4
Chinese tea, hot, loose leaf tea served in a big pot, originated thousands of years ago (legend has it that it was discovered when tea leaves accidentally landed in hot water) and is so integral to dim sum (teahouses actually predate the advent of dim sum) that 飲茶 yám chàh (Cantonese for drink tea) also means to go out for dim sum.
Varieties
There is a huge variety of Chinese tea, which can be split into categories such as green, black, white, scented... Jasmine and oolong tea is standard, and chrysanthemum is a common non-caffeinated option, but large Chinese restaurants usually offer many more (I once asked “what tea do you have?” and they responded “what do you want?”
Books
A Little Tea Book by Simon Beckwith describes the varieties of teas (not just Chinese), their characteristics, and proper preparation. The Dim Sum Field Guide by Carolyn Phillips lists teas commonly served with dim sum.