git/Documentation/glossary.txt

object::
	The unit of storage in GIT. It is uniquely identified by
	the SHA1 of its contents. Consequently, an object can not
	be changed.

SHA1::
	A 20-byte sequence (or 41-byte file containing the hex
	representation and a newline). It is calculated from the
	contents of an object by the Secure Hash Algorithm 1.

object database::
	Stores a set of "objects", and an individial object is identified
	by its SHA1 (its ref). The objects are either stored as single
	files, or live inside of packs.

object name::
	Synonym for SHA1.

blob object::
	Untyped object, i.e. the contents of a file.

tree object::
	An object containing a list of blob and/or tree objects.
	(A tree usually corresponds to a directory without
	subdirectories).

tree::
	Either a working tree, or a tree object together with the
	dependent blob and tree objects (i.e. a stored representation
	of a working tree).

cache::
	A collection of files whose contents are stored as objects.
	The cache is a stored version of your working tree. Well, can
	also contain a second, and even a third version of a working
	tree, which are used when merging.

cache entry::
	The information regarding a particular file, stored in the index.
	A cache entry can be unmerged, if a merge was started, but not
	yet finished (i.e. if the cache contains multiple versions of
	that file).

index::
	Contains information about the cache contents, in particular
	timestamps and mode flags ("stat information") for the files
	stored in the cache. An unmerged index is an index which contains
	unmerged cache entries.

working tree::
	The set of files and directories currently being worked on.
	Think "ls -laR"

directory::
	The list you get with "ls" :-)

checkout::
	The action of updating the working tree to a revision which was
	stored in the object database.

revision::
	A particular state of files and directories which was stored in
	the object database. It is referenced by a commit object.

commit::
	The action of storing the current state of the cache in the
	object database. The result is a revision.

commit object::
	An object which contains the information about a particular
	revision, such as parents, committer, author, date and the
	tree object which corresponds to the top directory of the
	stored revision.

changeset::
	BitKeeper/cvsps speak for "commit". Since git does not store
	changes, but states, it really does not make sense to use
	the term "changesets" with git.

ent::
	Favorite synonym to "tree-ish" by some total geeks.

clean::
	A working tree is clean, if it corresponds to the revision
	referenced by the current head.

dirty::
	A working tree is said to be dirty if it contains modifications
	which have not been committed to the current branch.

head::
	The top of a branch. It contains a ref to the corresponding
	commit object.

branch::
	A non-cyclical graph of revisions, i.e. the complete history of
	a particular revision, which does not (yet) have children, which
	is called the branch head. The branch heads are stored in
	$GIT_DIR/refs/heads/.

ref::
	A 40-byte hex representation of a SHA1 pointing to a particular
	object. These are stored in $GIT_DIR/refs/.

head ref::
	A ref pointing to a head. Often, this is abbreviated to "head".
	Head refs are stored in $GIT_DIR/refs/heads/.

tree-ish::
	A ref pointing to either a commit object, a tree object, or a
	tag object pointing to a commit or tree object.

tag object::
	An object containing a ref pointing to another object. It can
	contain a (PGP) signature, in which case it is called "signed
	tag object".

tag::
	A ref pointing to a tag or commit object. In contrast to a head,
	a tag is not changed by a commit. Tags (not tag objects) are
	stored in $GIT_DIR/refs/tags/. A git tag has nothing to do with
	a Lisp tag (which is called object type in git's context).

merge::
	To merge branches means to try to accumulate the changes since a
	common ancestor and apply them to the first branch. An automatic
	merge uses heuristics to accomplish that. Evidently, an automatic
	merge can fail.

resolve::
	The action of fixing up manually what a failed automatic merge
	left behind.

repository::
	A collection of refs together with an object database containing
	all objects, which are reachable from the refs. A repository can
	share an object database with other repositories.

alternate object database::
	Via the alternates mechanism, a repository can inherit part of its
	object database from another object database, which is called
	"alternate".

reachable::
	An object is reachable from a ref/commit/tree/tag, if there is a
	chain leading from the latter to the former.

chain::
	A list of objects, where each object in the list contains a
	reference to its successor (for example, the successor of a commit
	could be one of its parents).

parent::
	A commit object contains a (possibly empty) list of the logical
	predecessor(s) in the line of development, i.e. its parents.

fetch::
	Fetching a branch means to get the branch's head ref from a
	remote repository, to find out which objects are missing from
	the local object database, and to get them, too.

pull::
	Pulling a branch means to fetch it and merge it.

push::
	Pushing a branch means to get the branch's head ref from a remote
	repository, find out if it is an ancestor to the branch's local
	head ref is a direct, and in that case, putting all objects, which
	are reachable from the local head ref, and which are missing from
	the remote repository, into the remote object database, and updating
	the remote head ref. If the remote head is not an ancestor to the
	local head, the push fails.

pack::
	A set of objects which have been compressed into one file (to save
	space or to transmit them efficiently).

pack index::
	Contains offsets into a pack, so the pack can be used instead of
	the unpacked objects.

plumbing::
	Cute name for core git.

porcelain::
	Cute name for programs and program suites depending on core git,
	presenting a high level access to core git. Porcelains expose
	more of a SCM interface than the plumbing.

object type:
	One of the identifiers "commit","tree","tag" and "blob" describing
	the type of an object.

SCM::
	Source code management (tool).

dircache::
	You are *waaaaay* behind.