The first armor was nothing more than the clothes one wore. Although not very effective, a measure of protection was provided from slight scratches and bites. (we may not see this as much, but remember this is before antibiotics, the slightest infection could lead to death)
Plant materials could be used to make armor, in China some people used a type of wicker-worked armor and shield. Wood strips could also be used. Bones also made a natural protective material. Most armors of this type were made of strips of the material tied or sewn together to cover particularly vulnerable body parts; such as the stomach and chest.
With metal came a new material for armor. Copper and then bronze could be used to make armor, and frequently were. Again, this started as strips sewn or tied together, such as the Babylonians would have worn. As the skills involved with metal grew, so did the pieces of armor�. First the helmet and breastplate, again covering the most vulnerable parts. The Heroic age saw such armor, the Greeks of Homer and their ilk.
Somewhere in this period, and there is much debate by scholars over when and where, chain began to be used. Rings of metal were sewn into clothe or leather at first, then someone decided to link the rings directly together. This has always seemed to me a non-linear step in the evolution of armor, but whatever made it happen, it stayed the most widespread form of armor for centuries if not a millennium or more. The Romans, known for the Lorica Segmentata used chain(mail) armor more often and for a longer period of time. (note: Mail means chain, and so the term �chain mail� annoys some �armor snobs��. however, as it is common parlance in most recreational circles, I see it as just as legitimate)
Individual links of chain could be riveted or welded in order to protect against splitting, and this was the most difficult part of making chain. In fact, the simplicity of chain armor is most likely the reason it retained its prominence from the fall of the Roman Empire ca. 500AD to the common use of plate armor some 800 years later. Chain armor was very time-consuming� a single hauberk (usually a long sleeved chain shirt reaching the knees) could take 700 or more man-hours to make. Indeed whole towns could be mobilized to make chain armor supervised by a specialist.
Chain is most effective against a �slashing� attack�. That is, where the cutting surface connects with many rings. A thrusting attack, by a specialized weapon, would be interfered with by few or even one ring, causing catastrophic failure of the armor�s integrity. Chain also had limited to no use against a blunt attack. The supple nature of the armor means that the impact was directly applied to what was under the armor, with very limited absorbance. Most chain was backed by extensive padded garments, which both protected from impact, and gave the chain some absorbance value that it intrinsically lacked.
With the opening of trade between the East and West, coupled with the cultural effect of the Crusades, new ideas and technologies were opened in Europe. (add in the inevitable refinement of metal-working technique) In the 1300�s we start to see use of extensive pieces of plate armor; that is sheets of metal, shaped and linked to form consistent protection. This started, counter to the usual method, on the arms and legs instead of chest. Helmets also started to change, gaining a visor to assume the protective value of the large �great-helms� while avoiding the size and weight.
By the early 1400�s we see full suits of armor, head to foot, made of plates of metal on a more regular basis. This could be seen as a response to the use of missile weapons; longbows, crossbows and their ilk. As I stated before a thrusting or piercing attack can defeat chain armor, and arrows certainly fit this pattern. Plate armor, on the other hand, spreads the force of an attack over a greater surface negating its effect, or deflects it at an angle advantageous to the armor.
This armor hit its hey-day around the turn of the 16th century, with �Gothic� armor turning into �Maximilian� and �transitional� armors. Around this time firearms came into common use, and many see this, mistakenly, as the end of the usefulness of armor. Indeed, there are reports of mercenaries pulling knights off their horses and having to pull apart the armor in order to kill them, as their firearms could not pierce it even at point blank range.
The fluting, or ridges put into armor, increased their tensile strength without adding to the weight, however it also is technically difficult to pull off well, and so most armors simply became thicker. An armorer would shoot his breastplates in order to put in a �bullet� proof (proof of value against a bullet) and armor would often not be bought without this. (although it would not be difficult to �proof� an armor using a low-charge shot)
Eventually, in order to retain usefulness against better and better firearms armor became so thick and cumbersome as to negate its effective value. It is in this time period where we see horsemen being hoisted onto horses using cranes. The cost to value ratio of armored horsemen went south somewhere in the 1300�s. The longbowmen that were used to such effect in the 100 years war, were used because they were cheap, and filled out an army. The knight, kept its place, at first, unnaturally in order to maintain the noblesse of the class, but as war is an exercise for the pragmatist they were forced to change. They became the gentlemen officers of Europe, and breeding was often seen as important, or more so than experience or talent.
Back to armor. Specialized armor for the joust or tournament was made fairly early in the plate armor age, and continued until armor fell out of vogue. There are types of armor for different kinds of mounted joust, with or without a barrier, with or without a shield, with or without a grand guard, :you get the idea.
Armor specifically made for ground combat was also used, this a sign of the change in the value of the armored horseman, as at this time most knights dismounted in before battle because a horse was simply one more vulnerable spot to missile weapons.