Weapons are items which you can wield to improve your ability to attack enemies. In most cases they deal non-elemental damage, making them suitable for dealing with any threat (though they are subject to AC). Branded weapons may deal additional elemental damage as well, or have other effects. Weapons are divided into two categories:
Simply equipping yourself with the biggest, heaviest weapon you find does not guarantee that you'll be doing particularly good damage; your weapon's effectiveness is based on many factors. Weapon damage and weapon speed are the two most important aspects, and both of these will get better as your associated weapon skill improves. Your weapon's enchantment bonus is also critical - it would take a tremendous amount of skill to make up for a -9 enchantment penalty to your accuracy and damage, but a +9 weapon might be well worth using even in the hands of an unskilled fighter (though a skilled fighter would certainly make the best use of it). You should also consider the handedness of your weapon. A one-handed weapon will leave your off-hand free, allowing you to wear shields. Two-handed weapons prevent the use of shields, but are generally much more devastating. All of these factors are important when deciding what to fight with; see the weapon choice article for further details.
Weapons are by no means necessary to defeat your opponents. Unarmed combat specialists can be every bit as effective using only their fists, fangs, talons, and tails, though some species are significantly better at doing so than others. Magic-users, meanwhile, can deal tremendous amounts of damage through use of spells. Finally, there are magical staves - magical devices that are wielded like weapons, but which are treated by Crawl as a separate class of item entirely.
Prior to 0.14, there were elven, dwarven, and orcish variants of weapons which had small additional benefits, and demonspawn characters received a damage bonus when using demonic weapons - see Racial weapon variants. Also, several weapons were renamed: sabre to cutlass, double sword to bastard sword, and triple sword to claymore.