Are you talking about "Factions" ? It's not really an expansion, it's a new/seperate game. You can probably buy Factions FIRST , without ever having Prophecies. Prophecies is however the better one to learn the game as it is slower to start of, Factions' campaign is much faster.
Factions' also is not an "extension" campaign in the sense that you take your fully buffed up lvl 20 character from the original game and continue with that campaign. It's a new campaign meant to be played from lvl 1.
Prophecies [the original] got a single player/coop campaign in a certain world with a theme [the typical medieval theme], a few classes to pick from a few arena types for PvP etc.
Factions got ANOTHER single player/coop campaign in an entirely different world with another theme [very asian theme], ALL the classes from Prophecies plus 2 more classes and then all the usual goodies you would get in an expansion.
If you have both, you get more character slots and you can "eventually" move your character between the campaigns aka "worlds" of Prophecies/Factions if you complete a campaign.
You will also notice Prophecies and Factions will both cost full price [i.e. R300] . Which also indicates that they are almost independent games.