I'm no Chinese expert, so somebody correct me if I'm wrong, but I've found that the Chinese word 茶 (cha) doesn't always necessarily mean tea, but can refer generically to a number of different brewed drinks. e.g. barley tea (大麥茶), ginger tea (薑茶), golden oats tea (燕麥茶), etc. all of which translate to tea, but often contain no tea leaves. It may seem like a nitpick, but when you're in China and order what you expect to be a ginger flavored tea, only to receive a cup of hot water with chopped ginger at the bottom, the distinction can be important. That isn't to say you can't simply order 茶 in China and receive what you would expect, as long as you're expecting green tea. Likewise, if you simply order tea in England, you'll likely receive what the Chinese call 紅茶 (red tea). So in my mind, the words aren't exactly equivalent and I wonder how much the different variations of tea and cha relate to themselves and each other.

Edit: Applied jpatokal's correction.