According to Croteam, demo includes:
Serious Sam HD: The First Encounter is also available on the Xbox 360 through Xbox Live Arcade. Another high-def revamp is on the way in the form Serious Sam HD: The Second Encounter, planned for release early this year on PC and consoles.
One Single-Player Level - Get a taste of the intense action by battling through a specially designed demo level set in sunny Karnak with redesigned gameplay and changed-up enemy positions for unique battles. This demo level is so big it could take hours to get through each of the difficulty levels! Cooperative Mode Enabled! - Fight through the Karnak demo level with up to 16 players and finally get a chance to see classic Serious Sam coop in all its visual glory! Deathmatch Mode Enabled! - Step back into the glory days of frantic deathmatch with up to 16 players in the visually upgraded Desert Temple map.
I've been replaying this classic with a friend over Steam and having a blast! Can't wait for Second Encounter HD!