What on earth is a B777-200? And what does ER stand for?
Airlines generally tell you which type of aircraft you are flying on when your booking is finalised. Somewhere on your confirmation document it will refer to the aircraft name, sometimes under the word "equipment". Airline guides such as OAG also use this term and will tell you which sort you are likely to be flying on.
As on Seatplans.com, the aircraft type will be abbreviated, with, for example, the letter A standing for Airbus and B for Boeing. Behind this will be the numbers that signify what kind of aircraft it is, e.g. a B747, commonly known as a Jumbo Jet.
Just to make it a bit more complicated, after the first few numbers there may be a dash with some more numbers and perhaps a letter, such as B747-400 or B777-300ER. The reason for this is that like car manufacturers, plane makers modify, stretch and shrink basic models to suit different requirements.
So, a B777-300 is slightly different from a B777-200, while a B777-300ER has Extended Range, i.e. bigger fuel tanks to carry it further, although this may mean it carries slightly less passengers. It all depends on what individual airlines require for specific routes.
Generally speaking, the higher the number after the dash, the more up to date the variant of the aircraft.