Medieval Armour - A brief guide
First Part: BasicsAh.... the knight in shining armour - a long standing example of the importance of medieval armour.
A knight in medieval times wouldn't wear "shining armour", which is only found in romantic books, songs or fairy tales. The knights armour wasn't made "to look nice", it was made to protect, which is almost a miracle as the armour itself was barely a few millimetres thick. Typical medieval armour would consist of many parts, the most common found in the picture below.
Armour was often decorated by nobility to show their status. Some European rulers had armour made thinner and then covered in gold, just to display their wealth. Contrary to popular belief, armour was very costly, so only those who came from rich families could afford a full set. Some knights-to-be couldn't afford armour so they would remain as squires.
Second Part: Early Middle AgesMedieval armour was first made up of chainmail and a helmet. Such armour was common in western Europe but very rare in other parts of Europe, such as Scandinavia or eastern Europe. There, armour was made of hard leather or thick wool. Dark iron or leather helmets were a typical sight. Steel was also used but rarer. While leather could protect from many blows, chainmail was much better. Armour was often covered with cloth tabards or tunics, to show that the wearer was related to a country or an order.
Shields can be counted as armour too and were often large & heavy to offer protection from projectiles. The Norman "upside-down tear" type of shield was common in Normandy and England in the early middle ages. Other countries had circle or rectangle shaped shields.
Third Part: High Period (or "The time between the early and late middle ages")Armour in this age became larger, thus covering most of the body. The concept of a cuirass was invented. Knights were well protected from the weapons of those days, however armour sometimes was made too light or too heavy and it wasn't very flexible. Shoulder and chest pieces were the first to appear, then some leg armour such as vambraces and gauntlets. We refer to such armour as "partial-plate" as it does not cover the entire body. Most knights still wore early middle age armour, as it was much cheaper to produce than a partial plate set, however, partial-plate sets became increasingly common towards the late middle ages (which was why the armour was advanced to full-plate sets).
Shields were still made of wood, often reinforced with iron and covered with cloth. Some shields were made of iron.
Fourth Part: Late PeriodArmour began to become increasingly larger and covered the entire body from head to toe. This armour was much costlier than any other type, so a full plate set was extremely rare. Many knights still preferred the old-fashioned chainmail and cuirass. When the Late Middle Ages were coming to an end, full plate sets were increasingly common, as many knights were rich. Steel was the most common metal used in those times and only peasants wore leather.
Shields were now made of metal but wooden shields were still a common sight for infantry.
Fifth Part: After the middle agesThis is a little off-topic. However it fits in as an addendum. After the middle ages armour was simplified. Cuirasses were the most often used. Infantry still had cuirasses for a long time, until they became useless as guns became the common armament of the army. Cuirasses for cavalry disappeared for some time and only re-appeared around the Napoleonic wars. Those cuirasses were much thicker and could hold a musket ball, if launched from a distance. Shields disappeared completely from the battlefield.
After World War I, all armour was useless as it could not stop bullets and machine guns or shells from artillery. Helmets, however, became basic army equipment.