AR's are basically a semi-auto T/C Encore. On the Encore, you swap barrels to change calibers. On the AR, you change the upper receiver.
AR's have two main sections, the upper and lower receiver. The upper receiver is made up of the bolt carrier group, chamber, barrel, forearm... The lower houses the trigger group and stock.
Example;
If you already owned .223/5.56 AR, and you wanted one in a 6.5, you would need to purchase an upper receiver chambered in 6.5. You would then put the lower from your .223 onto the 6.5, and presto, complete rifle. Although, if it were me, I would purchase another lower so that I would have a complete rifle. Just my ¢.