The playable demo features two campaign missions and one custom game mission. You get two maps from the Haven and Inferno campaigns, and the '843 YSD: Falcon's last flight' single-player map. Two playable races are available to you and six of the 18 heroes in Dual Mode are open for selection.
The demo is a little under 700MB and you can find a list of download locations over at the game's official website. The demo should provide a short and hopefully sweet taster of things to come. Heroes of Might and Magic V is due for release on the PC in May.