Even more venerable than the spear, the stick is almost certainly the first weapon used by humanity. Only the stone is possibly older.
But, much like the spear, the stick suffers nothing in usefulness for its advanced age.
Master George Silver, writing ~1600, was of the opinion that the stick (the quarterstaff, in his case, a monstrous 8' ash shaft) was the "best weapon against all manner of weapons".
The form of the stick varies perhaps the most amongst weapons. In its simplest form, it is simply a section of tree branch, perhaps two feet in length.
At its most advanced, we have the English quarterstaff (also known as the short staff), 6-8' in length, with steel or iron caps on the ends.
Other variations are the long staff, 12' in length; the walking stick (an exemplar being the Irish shillelagh), 2-3' in length with a weighted handle; and shorter staffs such as the 4' stick used in Jogo do Pau.
So, there is a great deal of variation in the stick - probably more than any other weapon.
Luckily for us, each type of stick weapon is similar to another weapon we've already discussed. The quarter staff is very similar to the spear. The shillelagh is similar to a mace. Short staffs are like inferior swords. The long staff is like the pike (which I have not discussed, as it is only relevant to formation combat). So, we can simply treat the differences between each type of stick and its closest relative.
The quarterstaff is superficially similar to the spear, and there are similarities in their fighting techniques. However, the quarterstaff is typically somewhat longer than the spear, and it is typically gripper near the back end (as opposed to the more typical "half-staff" grip used on the spear, similar to the "half-sword" grip on the longsword).
The primary attack of the staff is the thrust, and for those doubting the power of a thrust from a staff, imagine a punch with more than double the power behind it being concentrated down to a steel or hardwood disk half the size of a fist. It's clear that a blow from a staff can be easily four or more times harder than a punch - easily enough to KO someone, collapse their windpipe, stun them, or rob them of their balance.
The staff can also strike, whipping the end quickly to hit the head, using the hips to generate power. The longer the staff, the more powerful the strike (but also the more power needed to generate the strike). The staff can perform all of the strikes the swords can - downwards, upwards (useful for groin shots), and side-to-side.
George Silver recommends using a combination, especially against a swordsman, as he finds that a swordsman has trouble defending against a thrust followed quickly by a strike (or vice versa).
A shillelagh is basically a big mace.
The Shorter Sticks
The main thing to remember when dealing with short sticks (2-4') is that, while they may be used much like swords, their capability for wounding is much less, and more attention needs to be paid to the continuation of the fight after a blow, as one blow will almost certainly never end the fight.
Silver is a big fan of the quarterstaff, and he is of the opinion that a man with a staff could best two equal opponents armed with swords, due to the massive reach advantage.
As with all weapons, an important part of staff fighting is grappling, perhaps even more so with stick weapons, as they lack the sharp parts that discourage the opponent from grabbing the end of the weapon. If the opponent grabs your staff and you can't get it back, you need to be immediately prepared to close to the grapple or flee. Or draw another weapon, although now your opponent has your staff, which is likely to be superior to anything else you are carrying!