Just in case you were confused, they're not the same thing. Here are some interesting tidbits about tea time, the high and the low:
Teatime originated with the Duchess of Bedford (1788-1861) who needed a little pick-me-up around 4 in the afternoon to fill that long stretch between the morning and evening meals. At first, she snuck her tea snacks, but then she wised up and invited her friends, and made it the fashion. My guess is somebody caught her sneaking cookies and she had to quick! pretend that it was the elegant thing to do all along.
(I also read somewhere that fashionable ladies didn't eat much at regular meals, due to their clothing, corsets and whatnot, and ladylike behavior restrictions that kept them to a nibble. So even if they could wait for dinner, they wouldn't ever have a fully satisfied appetite, and a little something in the afternoon would be just the thing.)
High tea was eaten at a regular high table, hence the term "high" tea. Afternoon tea was generally laid on small, lower tables, but obviously the term "low" tea didn't catch on. Too many class connotations, probably. Interesting that "high" tea was actually the dinner meal for the common people. Taken later in the evening (around 6-ish), you would usually get more substantial food at high tea than the dainty little crust-removed sandwiches and sweet cakes most people usually associate with the afternoon tea ritual.
Most places that serve "high tea" are actually serving a variation on the traditional English afternoon "low" tea, and you'll usually get small finger foods and sweets with your tea. I'm looking for a good online directory of places that serve high tea - will post more info when I find it.