Minecraft wiki entry for Demo_mode, associated text: The demo map is preset and always generates with the same seed: -343522682, corresponding to the text seed North Carolina.
This means that the player Spawn, Seed, and Generated structures are always the same.
The game is locked to Survival mode and cheats are off by default.
Cheats can be unlocked and other game modes can be accessible by using Open to LAN.
Multiplayer servers are inaccessible, although LAN multiplayer is allowed, with the limitation that only 1 demo account is allowed on a server (this is because the default Minecraft name ('Player') cannot be changed).
The game time is limited to five in-game days (1 hour and 40 minutes real-time or 100 minutes). After the time runs out, players can still navigate through the world, but not interact with it. Resetting the demo world also resets the demo timer.
Changing the in-game time forcibly with commands or external programs can reset the timer.