Structures, the Vanilla Minecraft Schematics

Whenever someone wanted to share a Minecraft build, it was done by either sharing the Minecraft world for someone to load up and look around at, or is a schematic file for pasting into an existing world. Schematics were largely more popular, since servers could use it in combination with WorldEdit or MCEdit to paste it into their existing server world. The schematic file format has been around for a very long time, the first occurrence I can find in MCEdit's source being in October 2011 (though it probably existed before that).

Schematics were never endorsed by Mojang, so they required 3rd party tools to use. There isn't any specific rules of what a schematic should be, though they were generally decided by how WorldEdit or MCEdit used them. The current documentation of the format can be found on the Minecraft wiki. It includes a lot of the different fields added by community tools or outdated versions of MCEdit.

Around when 1.7 was released (2013), Mojang started to move away from numeric IDs deciding what blocks existed. This was mainly due to the fact there was a limit of how many different blocks could exist, but also because it's generally bad practice to use number IDs for data. This meant they moved from "1" being a stone block in the world to "stone" being a stone block. They started enforcing this with the vanilla commands, despite the fact numeric IDs were still used internally. In 1.12, you were no longer able to use numeric IDs in commands, and in 1.13 what was referred to as The Flattening happened. 

In combination to numeric IDs, pre 1.12 versions used data IDs. They were used when there was a block type, but it had sub types. For example, you had the wood log block, but there were about six variants of that block. So for oak wood, you had wood:0, birch had wood:1, etc.In 1.13, however, they removed all of these sub-type IDs and replaced with their own dedicated materials. This meant instead of wood:0, you had oak_wood, birch_wood, etc. They then added tags to these groups to make them easily discernable without having to do a ton of extra checks.

In 1.9 (released three years ago), though, Mojang added the officially supported and used Structure file formats. They were used internally to generate some of the natural structures, for example the End Cities and end ships. At the same time, they added the user-accessible Structure Block, which allowed players to select and save areas to a file and load it up again later. Despite the fact they were officially supported and part of the game, it never seemed to catch on and seemed to be largely ignored. WorldEdit didn't add support for them, and Minecraft build databases didn't support structures as a download option. 

With the new 1.13 update removing numeric IDs, hopefully Structures will be making a comeback. The file format is clear, and supports things like entities and block NBT data. One of the main reasons it isn't really used in Spigot plugins (that instead use schematics) is because no maintained libraries support them. In an effort to fix this, I'm trying to contribute to the Spigot project (and exploiting the fact that it's open source) by adding them into the Bukkit API for use by plugins without 3rd party libraries.