The Demoman is for the player who likes to pack a very large amount of firepower. The Demoman's primary weapon is his Grenade/Pipe Launcher, which has two separate fire modes: timed detonation and remote detonation. The remote detonation pipe bombs make the Demoman excellent at defending important areas. His weapons have a lot of explosive power, the trade-off being a lack of serious armor, moderately slow speed, and the fact that his Grenade Launcher requires practice to master.
Although he is primarily a defensive class, the Demoman can function in an offense role as well. Even though his health and armor are not the greatest in the game, the sheer power of his weapons puts the Demoman into the "heavy" category of offense: ideal for punching holes in a strong enemy defense to allow lighter attackers through.
The standard-issue crowbar for every class. It does some damage, but not enough to make a difference in a battle against a full-health opponent. It's a good humiliation tool for someone who is about to die, however.
The pelletgun is a weapon of last resort. Use it against opponents that you know you won't be able to hit with your blue grenades and pipebombs, or if you find yourself caught with your grenade launcher unloaded. It can also be used to save grenade ammo; Scouts that have been hit with a blue grenade will die in one pelletgun shot.
The Grenade Launcher holds six blue grenades, which have a hybrid timer/contact detonation system. If you score a direct hit on an opponent, it will do a large amount of damage. If the grenades don't score a direct hit, they explode automatically in just a little over a second after being fired.
This is an alternate fire mode of the grenade launcher, so it shares the same clip. Pipe bombs are yellow and feature a remote detonation system. You can lay up to eight of them; they are detonated using mouse2. Pipe bombs will explode automatically if the Demoman who laid them dies, if they are not detonated for a few minutes, or if you lay more than eight. There is a period of about a second after firing a pipebomb, where it glows red and cannot be detonated.
A standard fragmentation grenade. Use this when you have run out of the more powerful MIRV grenades (below).
The most destructive grenade in Fortress Forever. The MIRV grenade explodes at the end of its fuse, releasing four more mini grenades which then explode too. That's 5 high-damage explosions for your enemies to avoid.
Additional Strategy and Tips
Playing Against a Demoman