"METS is a widely used XML standard developed at the Library of Congress for encoding metadata about digital objects for use in digital libraries. It permits the description not only of individual digital items but also of groupings of digital files that together constitute a single research object (for instance, a TEI transcription of a poem, an audio file of someone reading the poem, and an archival finding aid describing the manuscript collection from which the poem is taken). It is a very useful standard for describing and managing large digital collections. Most scholars will not need to work with METS directly, but they should be aware of its existence, and will find it useful to understand its place in the digital universe when working closely with members of the library community." -- Definition taken from Encoding Guide for Early Printed Books Published by Brown University Women Writers Project
Examples of METS documents can be found here
At the most general level a METS document may contain the following sections: each of which is described in its own section of this chapter.
METS Header – The METS Header contains metadata describing the METS document itself, including such information as creator, editor, etc.
Descriptive Metadata Section – This section contains descriptive metadata that is external to the METS document (e.g., a MARC record in an OPAC or a MODS record maintained on a WWW server), internally embedded descriptive metadata, or both. Multiple instances of both external and internal descriptive metadata may be included in the descriptive metadata section.
Administrative Metadata Section – Information about how the files were created and stored, intellectual property rights, metadata regarding the original source object from which the digital object was derived, information regarding the provenance of the files that comprise the object (i.e., master/derivative file relationships, and migration/transformation information) is collected this section. As with descriptive metadata, the administrative metadata can be either external to the METS document, or encoded internally.
File Section – A list of all files that contain content which make up the electronic versions of the digital object. File elements may be grouped within File Group elements, to provide for subdividing the files by object version or other criteria such as file type, size etc.
Structural Map – This is the heart of the METS document. It outlines a hierarchical structure for the digital object, and links the elements of that structure to content files and metadata that pertain to each element. The structural map is the one mandatory section in a METS document.
Structural Links – Allows the creator of the METS document to record the existence of hyperlinks between nodes in the hierarchy outlined in the Structural Map. This is of particular value in using METS to archive Websites or other hypermedia.
Behavior Section – A behavior section can be used to associate executable behaviors with the content of the object encoded using METS. Each behavior within a behavior section has an interface definition element that represents an abstract definition of behaviors represented by a particular behavior section. Each behavior also has a mechanism element that identifies a module of executable code that implements and runs the behaviors defined by the interface definition.