Plate Armor is, simply put, armor made of solid plates of metal, essentially a slab of metal made to be worn by people. The cumbersomeness and the lack of mobility is often exaggerated in media, mostly due to confusing tourney armor with combat armor. Tourney armor indeed was unreasonably heavy, thick and restricting in movement, precisely because no one wanted to die or just break a rib (which considering the shitty medicine of that days could be fatal as well) on a tourney, but when knights and men at arms went to war, they clad themselves in a much lighter combat armor, in which they could run, jump, or even perform a somersault more easily than chain mail (which had its weight distributed almost entirely on the shoulders) - because in a real fight you should fight for hours rather than few minutes, and mobility is just as important as armor for survivability (though that being said, funnily enough tests have noted that modern soldiers carry a similar weight on them to a full suit of plate armor. Their load is less taxing because more of it weighs down on their back instead of their legs). What we think of as Plate Armor began to take shape around the late 1300s and became what most people thought of as plate armor in the 1400s - partial plate armor has existed for centuries before that, with (albeit not steel) breastplates existing since classical antiquity, such as the Greeks' bronze muscle cuirasses.
A decent plate armored suit could deflect or shatter most blades and arrows that struck it due to its hardiness, requiring those who wielded these weapons to either find chinks in the armor to stab at the wearer (which is a bit hard when he's trying to chop your head off with impunity), or use concussive/heavy piercing weapons like maces and warhammers, which could either bypass or penetrate the plating and still at the absolute worst probably cause the wearer to be terribly shaken from the impact. Arrows had limited effect as well, although bolts shot from crossbows typically had enough power to at least dent plate armor, and heavy crossbows might do so while leaving the wearer bruised. Contrary to popular belief, guns didn't pierce through plate with ease - only high-caliber muskets had enough power in their shot to reliably pierce though the plate, and much like heavy crossbows they paid for it with their sluggishly slow firing rate. From the times of sword and shot also originate the term "bullet-proof", as when a noble or a mercenary went to buy a new set of armor (or just a plate cuirass), he got a pistol with him, and "proofed" his buy by shooting at it at point-blank - good armor suit were supposed to hold that shot, and good luck selling anyone a suit which hadn't passed the test, because who the fuck gonna buy and armor that couldn't stop a pistol in a times when everyone and his dog is armed with one. Unscrupulous smiths were known to use a round-ended hammer to create a fake bullet mark on their armour in order to sell poorly-made products to unsuspecting rubes. Even in Sengoku era Japan, where militaries absolutely spammed guns, most conscript soldiers had breastplates.
Fiction often portrays plate armor as the most expensive armor there is, but it really wasn't. By the 15th century (not that far after plate armor itself was invented) high demand and the invention of the automatic (water wheel powered) hammer resulted in "munitions grade" plate being produced in large quantities. Conversely, mail remained very labor intense to make and was entirely obsolete beyond armoring joints. Never the less, a suit of custom made full plate was well beyond what most people could afford back in the day. Even with modern day tools, getting a suit of armor is still expensive since it has to be made to order to fit your body, and you have to maintain your current body weight or else it won’t fit. Because of how a full suit is dependent on interlocking pieces, getting the dimensions just right is crucial. Munition armor was rarely ever full plate, usually limited to a cuirass and helmet, or a “half-plate” suit of armor that only covered the upper body.
Nonetheless, military history clearly moved away from plate armor, finding it logistically impractical to pay and supply bullet-resistant armor to their troops compared to just getting more men and guns, even before guns improved to an extent that no worn armor would have been able to stop bullets and still be light enough to run around in until kevlar was created. The factions of WWI experimented with putting body armor on their soldiers sufficient at protecting them against small arms, but as previously implied, found that stacking enough metal on a man to protect him from rifle bullets made it unfeasible for him to even move - the Germans adopted "Sappenpanzer" body armor to some extent, but reserved it to sentries and static machine-gunners that didn't risk exhaustion from use while only expecting it to protect against shrapnel...but at this point of time, with good-quality rifle-resistant body armor consisting of armor plates inserted into "carrier" vests, plate armor to some extent is back baby! Humanity just had to think up fancier materials than steel.
In fantasy, plate armor is typically given to warrior classes to denote their position as the front-line close combat fighters. It's commonly also the victim of Fantasy Armor syndrome where the armor is so bulky beyond belief that you start to wonder how the wearer is able to move, much less fight, when the armor is twice as big and heavy as him.
Parts of Plate Armor
- Plate helmet (many variants existed, although they're all designed to protect the head)
- Visor (Covered the face. Was usually hinged with the helmet, although some people did not bother with visors)
- Gorget (covers the entire neck)
- Bevor/Falling Buffe (covers front of neck)
- Spaulders (covers the shoulders)
- Pauldrons (an evolution of the spaulders. Pauldrons are larger than spaulders, not only covering the shoulder, but most of the upper arm as well. They're also reliable as blunt implements to shoulder charge somebody with.)
- Rerebraces (covers the upper arms)
- Couter (covers the elbows)
- Vambraces (covers the forearms)
- Gauntlets (covers the hands)
- Curiass (plate which covers the torso)
- Breastplate (single plate which covers the front of the torso)
- Backplate (covers the back of the torso)
- Tassets (covers the upper legs. They're the plate skirt that covered the cuisse)
- Cuisses (covers the thighs)
- Poleyn (covers the knees)
- Greaves (covers the lower legs)
- Sabatons (boots for walking and covers the feet)
Notable things that are not part of real world or realistic plate armor:
Plate armor by region
- Ancient Greece
- Medieval Europe