NBT

NBT (Named Binary Tag) is used all over Minecraft. The Minecraft Wiki has an extremely well put together page on the topic, so only the most relevant information will be placed here for ease of access.

Compression

In the context of Beta Minecraft, there are two relevant forms of compression that’re applied to NBT Data:

Format	Used for
GZip	Player Data, level.dat
Zlib/Deflate	Chunk Data

Tags are what NBT Data consists of. They typically start with a single byte describing their tag type/ID, followed by the value of that tag.

See Data types for more info.

Hex	Tag	Contains	Purpose
`0x01`	`TAG_Byte`	Byte	A signed byte, also used for booleans
`0x02`	`TAG_Short`	Short	A signed short
`0x03`	`TAG_Int`	Integer	A signed integer
`0x04`	`TAG_Long`	Long	A signed long
`0x05`	`TAG_Float`	Float	A signed float
`0x06`	`TAG_Double`	Double	A signed double

Hex	Tag	Contains	Purpose
`0x00`	`TAG_End`	`0`	Used to mark the end of compound tags. This tag does not have a name, so it is always a single byte 0. It may also be the type of empty List tags.
`0x07`	`TAG_Byte_Array`	Integer of array Length, followed by bytes	An array of bytes
`0x08`	`TAG_String`	Unsigned Short of string length, followed by UTF-8 Characters	A UTF-8 string
`0x09`	`TAG_List`	Byte for the Tag ID that’ll be used throughout the list, Integer of how many entries are in the List, followed by Tags without a tag ID.	A list of tags without repeating the tag ID header
`0x0A`	`TAG_Compound`	Can contain any number of tags, is terminated by TAG_End	Contain full formed tags. All tags inside need to have a unique name

Every NBT-formatted file starts with a TAG_Compound, and is terminated with a TAG_End.

Check the projects page for libraries that implemnt NBT in the language you’re working in.