As you can expect from the fact that Heathrow Airport is one of the largest and busiest airports in the world, there are four terminals and each terminal is gigantic. I have flown with British Airways and American Airlines in Terminal 5 and 3. Unlike other grand scale airports in the world, Heathrow's terminals are relatively complex with gates located in many side branches in the terminal 3. British's Terminal 5 is rather straightforward but there are two satellite terminals. I did not have a chance to transfer from one terminal to another as my destination was to/from London, but I can imagine that unless you are prepared, it will be hectic.