Slim_slag,
BA may have interline agreements and may even carry a few passengers who interline onto other carriers but far, far more relevant is that you can make online connections on BA. They will fly you from Prague to Chicago or from Stockholm to Jo'burg.
Which is why BA, Air France, Lufthansa etc refer to themselves as 'network carriers'. If you want to fly Ryanair from, say, Glasgow to Berlin via Stansted, you have to buy two tickets, pay two sector fares, check your bags twice - and you have no recourse if you miss your connection.
Apart from the obvious, that BA flies longhaul, almost all other differences between them are just a matter of degree.