There are five weapons in Assassasin's Creed:
Digital pad of Xbox 360 and PS3
a hidden blade
throwing knives/a short blade
Here is a description of weapons:
Using fists against armed guards is completely useless. You should use them only during interrogations.
A powerful weapon no self-respecting killer will leave home without. You can use it in any situation, when your target is not expecting an attack. For example: in "Save Citizen" missions you can kill the first enemy instantly, and then finish the rest of them with your sword. Don't expect the guard you want to kill is just going to stand dumbly like a sacrificial lamb. In addition, if you're using a hidden blade in High Profile mode, you'll be chased for sure. That's why you should use this kind of attack when you're the one chasing your target.
This weapon is very useful against guards and archers surveying the surroundings from building roofs or towers.
It is a useful weapon against all kind of enemies, even bosses, as almost every successful counter attack ends with enemy's instant death. A short blade can be used with throwing knives - when you are close to the enemy slice him with the blade and when the distance grows bigger, throw a knife.
That is your basic and most useful (in most cases) weapon .