In the standalone game one fights an individual battle and then when the round ends unless it's a clan game, that's it. What the Battlefield 2 Tour of Duty does is provide multiple ten hour campaigns that players can drop into as often as they like and the results of battles will affect the overall outcome of the wider campaign which will be directed by Generals, players who have risen up the ranks in the campaign system.
It's not the first time someone has jury rigged a game like Battlefield into a persistent campaign, though it is a highly welcome addition to Battlefield 2 should you be so inclined. See here for more. ยต