Every piece of data which comes through your telephone line, to your router, contributes to your download total. That includes emails, web pages, images, Youtube videos, video streaming (such as from the BBC iPlayer), audio streaming, program files, MP3s, game data, etc.
Some of those things (such as text-only emails) require very little data to be transferred. Other things (particularly video streaming) use massive amounts of data.
BT charge £5 for every 5GB by which you exceed your monthly 10Gb allowance. It's not clear from their website, however, whether you only pay an extra £5 when you've used a full extra 5Gb or whether (as is probably more likely) they really mean "5Gb or part thereof".
Chris