Note: All images are representations, not to scale. All the poles would be much longer. Also, these are the most basic representations, they varied from country to country, and even as time progressed they changed appearance, so these may not look how you expect them to.
A bit more complex than a simple spear or pike, but this is where the polearm originated from. As far back as the Stone Age (predating 1000 B.C.) polearms have been in use, beginning with the simple spear. They lasted centuries, right up until the last known polearm, the halberd, which did not become obsolete until late 1500s with the end of the medieval/beginning of the Renaissance period.
The concept of the polearm is simple: attach common weapon to end of long stick. In its first begginings, this was the common stick simply sharpened at the end, so that one could attack their enemies without entering a melee, and one also had the advantage of being able to deal out extensive damage to any charging enemy. This simply sharpened stick wasn't enough once armour became a consideration, and now, by around 600 B.C., the polearm began to evolve.
The first modern adaptation was to strengthen the wood, and then to attach a metal head, rather than simply sharpening it. This meant that it was less likely to break, and more likely to pierce something like leather armour. As time progressed, however, armour turned to metal to counter the fearsome strength of the spear, and so the spear yet again evolved, the shaft becoming metal, and the head becoming longer and thinner. Previously, it had been a fairly wide and stout head, so that it ripped through as much flesh as possible, but now it was made long and thin so that it had the power to puncture metal.
The polearm was rather stagnant for a while now, until the Medieval period began. Around 500 A.D., with the fall of the Roman Empire, peasants found themselves fighting for their own protection. This became more so later in the Feudal Age, around 1000 A.D., but nevertheless three key weapons became known. The first was the simple pitchfork, the second the scythe and finally the bill. All of these were used for turning soil and/or cutting crops, but when the usefulness of them was seen, they were quickly adapted to military equivalents.
The pitchfork evolved into the war fork, or military fork. It no longer had three or four spikes, but two which came around in a horse shoe, looking a bit like so:
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The way this was designed it had more puncturing power, and was stronger than your traditional pitchfork, but it was harder to follow through on attacks and pierce vitals. Nevertheless, it was still quite effective. The scythe, similarly, became known as the war scythe or military scythe, but it remained more or less identical to a common scythe, with the exception that the tip of the scythe was made stronger so it would puncture, and the pole was replaced with metal.
The bill became known as the bill hook, and was mainly used by the Britons. It was not an exceptional weapon, but it was the earliest response to mounted combat. It was adapted from the normal bill and added on to include not only hooking, but piercing and slashing capabilities. It could be used first to stab at horses, then to hook any riders that make it through, pulling them onto the ground, and finally stabbing them while they are prone. It looked a little like this:
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First the spear head, A, would be used to stab, then the hook, B, would be used to pull, and then the spike, C, would be used to kill when prone. The next evolution came from the spear, and like the bill hook, was a direct response the mounted combat. This next weapon was the pike.
Where the spear was used to stab and thrust from around 2m away, the pike was up to 6m long, and was designed to be held against a charge, rather than to charge with. The spear could be used in the same way, but not to the same effect. Cavalry, and many other unit types, had virtually no chance of surviving a direct charge, and it was incredibly effective when mixed with other units. As the medieval period progressed, however, new tactics were produced to overcome current polearms, and more needed to be developed.
One of these new developments was the guisarme, which combined a spear and a blade, and looked a little like this:
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The guisarme was difficult to use, but in well trained hands, it was deadly. Not only did it have the puncturing power of a spear, but the slashing damage of something like a scimitar, and still at a distance of around 3m. Another of these new developments was the glaive, which looked a little like this:
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It was, more or less, a large axe on the end of a pole, with a pointed head so that it could be used similarly to a spear. It had incredible hacking power, but was much less powerful as a spear. The next evolution of this weapon was the bardiche, which essentially was an axe head on a pole, without any of the added spear. It was a large, curved axe head and it was extremely deadly. The third, and final, evolution of this type of polearm was the deadly halberd. It reverted back to the same principle of the bill hook, but was far more effective. It looked a little like this:
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Like the bill hook, first the spear head, A, was used to stab at charging horses, then the hook part, B, was used to dismount any riders that got through and finally the axe head, C, was used to attack them while prone. Against opponents on foot, there were three choices. The spear head could be used to keep them at a distance, the hook could be used as a pierce attack or the axe head could be used as a very powerful hack attack. The halberd was the pinnacle of polearm technology, although it was quickly obsoleted by the emergence of gunpowder.
Despite this, the polearm was still used. While most were discarded, some such as the pike, were used throughout the Renaissance to guard musketeers from cavalry. By the time napoleonic warfare and more sophisticated, faster firing guns became available, the need to protection against cavalry was simply covered by gunmen.