The free version of the game does not include the campaign mode, HD graphics or support for user-made addons and mods. It does, however, include the full range of over 300 weapons, units and vehicles; the 50+ player multiplayer mode; support for play on dedicated servers from either the free or retail version; and the single/multiplayer mission editor. There are no microtransactions and no hidden costs in the game -- it's completely free, effectively making it the world's most generous demo.
Find out more at the and watch out for the game later this month -- and Arma 3 in the summer of next year.