Digital Combat Simulator
Freebies — what's in the box?
The free, base DCS World package comes with one full-sim aircraft module, the TF‑51D Mustang; one simplified-sim (“FC3-style”) aircraft, the Su‑25T “Frogfoot”; and one piece of terrain, the Caucasus region.
The modules
DCS content is divided into “modules.” There are generally four kinds: aircraft, terrain, add-ons, and campaigns. Aircraft modules are pretty much what it says on the tin: a single aircraft, occasionally with a few variants thrown in, and often accompanied by single campaign. Terrains are the world maps in which your mission take place. Add-ons are an odd mix of speciality content for specific purposes — there is no real coherent theme other than that they require some other module to really add anything useful. Campaigns, finally, are just that: a set of missions for a specific aircraft and terrain (having both as a prerequisite in order for the campaign to run).
Most aircraft modules also come with a built-in campaign as well as a collection of training and quick-action missions.