Basic Information
Like a spear only longer, the pike tends to start at a length of 10' or so. The pike crops up on a semi-regular basis throughout history, depending on the conditions on the battlefield at the time.
The first well recorded appearance of the pike is as the Macedonia sarissa, developed by Philip II of Macedon and leading to a revolution in ancient warfare. In some ways this was a logical outgrowth of traditional hoplite warfare, but its introduction completely revolutionised the structure and operation of the phalanx and it continued to be used by the Hellenistic states until they were replaced by Rome, who favoured sword armed heavy infantry instead.
After that, the pike crops up from time to time in the middle ages until the start of the gunpowder era, when it appeared to be the perfect counterpart to firearms - another easily mass produced weapon that could be deployed with fairly basic training, ideal for providing a defensive base from which the musketmen could lay down fire.
Actual usage varied considerably - some medieval and late classical users treated the pike as purely defensive, deploying them in large 'hedgehogs' to protect against enemy cavalry. In this way they provided a fortress of sharpened points that was hard to break into, but almost impossible to move. In many cases these formations were used to anchor other units of cavalry, missile troops or heavily armed light infantry who could retire within their protection if counter attacked. Hedgehog formations were also extremely vulnerable to missile fire due to their size, immobility and the poor quality (and therefore lack of armour) of the men inside.
More organised pike users could deploy them effectively in the attack - the Swiss were acknowledged masters of this, but the Scots were also well known users and the Germanic states later adopted the pike phalanx as well. This relied on a great deal of co-ordination - which was helped by the rediscovery of close order drill in the late medieval period and the growing professionalisation of warfare - and the standards of ancient Macedon were not always met. Where they were, as the Swiss proved, a well handled pike phalanx could roll straight over almost anyone.
There were, of course, downsides - the pike phalanx, no matter how well drilled, remained vulnerable to missile fire (as the Scots were repeatedly shown by English longbowmen) and artillery fire and coped extremely badly with uneven ground, wooded terrain and the like. Some of these vulnerabilties were addressed by the deployment of flankers (frequently armed with polearms or double handed swords) and the developement of pike-and-shot warfare (where the pikes were supported by missile troops). This probably reached its achme in the Spanish tercio formation which mixed pikes (about half the total strength of the unit), musketeers and/or crossbowmen and sword and buckler men (each category about half of the remaining strength).
The pike was rendered obsolete by a combination of improved muskets and the bayonet - a formation of men so armed were protected from cavalry attacks by the bayonet hedge and could generate a far higher volume of ranged fire than a force of equivalent numbers split between pike and musket.