Heathrow has four passenger terminals in operation: 2, 3, 4 and 5. Airline and route assignments differ by terminal.
Terminal 2
Terminal 2 is used by many Star Alliance airlines and other international carriers. It shares Heathrow's central rail/Tube area with Terminal 3.
Terminal 3
Terminal 3 handles a broad mix of long-haul airlines, including some British Airways services and many oneworld and independent carriers.
Terminal 4
Terminal 4 is separate from the central terminal area and has its own rail and Tube stations. Allow extra time if you accidentally travel to the wrong terminal.
Terminal 5
Terminal 5 is dominated by British Airways and Iberia operations. It has its own rail and Tube station and is not a short walk from Terminals 2 or 3.