texmffor the name of the root directory (folder) of an hierarchy; in practice there are typically several such trees, each of which has a name that compounds that (e.g.,
texmf-var). Files supplied as part of the distribution are put into the distribution’s tree, but the location of the distribution’s hierarchy is system dependent. (On a Unix system it might be at /usr/share/texmf or /opt/texmf, or a similar location.) There may be more than one ‘local’ hierarchy in which additional files can be stored. An installation will also typically offer a local hierarchy, while each user may have an individual local hierarchy. The TDS itself is published as the output of a TUG Technical Working Group. You may browse an on-line version of the standard, and copies in several other formats (including source) are available on CTAN.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=tds