With something like git.git current tree, I get:
[torvalds@g5 git]$ git describe parent
v1.0.4-14-g2414721
i.e. the current head of my "parent" branch is based on v1.0.4,
but since it has a few commits on top of that,
describe has added the number of additional commits ("14") and
an abbreviated object name for the commit itself ("2414721")
at the end.
The number of additional commits is the number
of commits which would be displayed by "git log v1.0.4..parent".
The hash suffix is "-g" + an unambiguous abbreviation for the tip commit
of parent (which was 2414721b194453f058079d897d13c4e377f92dc6). The
length of the abbreviation scales as the repository grows, using the
approximate number of objects in the repository and a bit of math
around the birthday paradox, and defaults to a minimum of 7.
The "g" prefix stands for "git" and is used to allow describing the version of
a software depending on the SCM the software is managed with. This is useful
in an environment where people may use different SCMs.
Doing a git describe on a tag-name will just show the tag name:
[torvalds@g5 git]$ git describe v1.0.4
v1.0.4
With --all, the command can use branch heads as references, so
the output shows the reference path as well:
[torvalds@g5 git]$ git describe --all --abbrev=4 v1.0.5^2
tags/v1.0.0-21-g975b
[torvalds@g5 git]$ git describe --all --abbrev=4 HEAD^
heads/lt/describe-7-g975b
With --abbrev set to 0, the command can be used to find the
closest tagname without any suffix:
[torvalds@g5 git]$ git describe --abbrev=0 v1.0.5^2
tags/v1.0.0
Note that the suffix you get if you type these commands today may be
longer than what Linus saw above when he ran these commands, as your
Git repository may have new commits whose object names begin with
975b that did not exist back then, and "-g975b" suffix alone may not
be sufficient to disambiguate these commits.