NBT format

The Named Binary Tag format is used by Minecraft for the various files in which it saves data. The format is described by Notch in a very brief specification. The format is designed to store data in a tree structure made up of various tags. All tags have an ID and a name. The original known version was 19132 as introduced in Beta 1.3, and since then has been updated to 19133 with Anvil, with the addition of the Int Array tag. The NBT format dates all the way back to Indev with tags 0 to 10 in use.

Another more user-friendly format of NBT is in plain string, as used in commands. This format is referred to as SNBT, short for stringified NBT. It is different from the JSON format; hence, any JSON used in NBT, such as raw json text, must be enclosed within a string tag.

TAG definition
A tag is an individual part of the data tree. The first byte in a tag is the tag type (ID), followed by two bytes for the length of the name, then the name as a string in UTF-8 format (Note TAG_End is not named and does not contain the extra 2 bytes; the name is assumed to be empty). The name of tags may contain spaces, although Minecraft itself never saves tags with spaces in the names. Finally, depending on the type of the tag, the bytes that follow are part of that tag's payload. This table describes each of the 12 known tags in version 19133 of the Named Binary Tag format: The List and Compound tags can be and often are recursively nested. It should also be noted that, in a list of lists, each of the sub-lists can list a different kind of tag.

File Format
An NBT file is a GZip'd Compound tag, name and tag ID included. Some of the files utilized by Minecraft may be uncompressed, but in most cases the files follow Notch's original specification and are compressed with GZip. In the Legacy Console Edition, chunks are compressed with XMemCompress, a variation of an LZX compression algorithm. There is no header to specify the version or any other information - only the level.dat file specifies the version.

As used in Minecraft
Minecraft's use of the NBT format is odd at times. In some instances, empty lists may be represented as a list of Byte tags rather than a list of the correct type, or as a list of End tags in newer versions of Minecraft, which can break some older NBT tools. Additionally, almost every root tag has an empty name string and encapsulates only one Compound tag with the actual data and a name. For instance: Another noticeable oddity is that, although the original specification by Notch allows for spaces in tag names, and even the example uses spaces in the tag names, Minecraft has no known files where any tags have spaces in their names. There is also inconsistent use of letter case, mostly either camelCase or PascalCase, but sometimes even in all lowercase.
 * The root tag for most Minecraft NBT structures.
 * : The only tag contained within the root tag - it has a name and contains all the actual data.

Uses

 * level.dat is stored in compressed NBT format.
 * .dat files are stored in compressed NBT format.
 * idcounts.dat is stored in uncompressed NBT format.
 * villages.dat is stored in compressed NBT format.
 * map_<#>.dat files are stored in compressed NBT format.
 * servers.dat, which is used to store the list of saved multiplayer servers as uncompressed NBT.
 * Chunks are stored in compressed NBT format within Region files.
 * scoreboard.dat is stored in compressed NBT format.
 * Generated structures are stored in compressed NBT format.
 * Saved structures are stored in compressed NBT format.

Software
Mojang has provided sample Java NBT classes for developers to use and reference as part of the source code for the McRegion -> Anvil converter. Since 1.13, Minecraft includes a builtin converter between SNBT format and compressed NBT format. It comes with the client and the official server.

Official
The data generator from Minecraft is able to convert uncompressed Stringified NBT files with .snbt extension in an input folder to GZip compressed NBT format files with .nbt extension in an output folder, and vice versa.

The vanilla data generator can convert any GZip compressed NBT format to SNBT format. You can simply change the file extension of a file, such as level.dat  to level.nbt and put it in the input folder, and the generator will decode the GZip compressed NBT data.

Third-Party
The community has developed programs to view and modify compressed and uncompressed NBT files: