As a single player you have three choices from the start, not including what you can do with that terrific editor!
Armed Assault comes with a series of 6 missions just for familiarization and learning the game. It starts with the obstacle course, then the weapons range, then to the armored vehicles, command of troops and ending with two helicopter tutorials. All are worth doing and very well done… except for the leadership mission which is known to not work correctly every time because your troops won’t fire on the targeted car you instruct them too.
Then you have a selection of 12 missions that you can jump right into. Well, kinda… The first four are open and unlocked. You have to complete them to unlock the others. I personally disagree with this approach. I hate having to unlock stuff in computer games unless it’s a campaign and / or a story mode type of situation. The missions are pretty good, and demonstrate the capabilities of the game well. But they can be hard at times, and usually unrealistic. They usually place you alone or with a small group of fellow AI soldiers against outrageous odds without proper support or equipment. You will be re-doing these missions a lot before you finally get lucky and succeed in one.
Armed Assault comes with a single player campaign. You go from start to finish. Unlocking other missions, and other story events along the way. The campaign is well done and has a few new features, like the ability to pick which direction you go in the story. Say for instance, you might get an opportunity to play as a sniper and ambush an infantry convoy en-route to reinforce parts of the battlefield. You can play the mission, making the campaign easier later on, or skip that option and have to deal with more infantry later. It gives the campaign a dynamic feeling to it, thou it really is not. There are a lot of rough spots in the campaign, but it is completely able. You’re brought along in the story through a series of cut scenes. Most are done really well. But there are minor annoyances and problems with the campaign. Mostly due to unrealistic situations or too difficult objectives. I’ll let you learn that for yourself.