The description of how LaTeX font attributes are turned into TeX
fonts is usually kept in a font definition (.fd) file. The
.fd file for family <family> in encoding <encoding> is called
<encoding><family>.fd, for example ot1cmr.fd for Computer Modern
Roman or t1ptm.fd for Adobe Times.
Whenever LaTeX encounters an encoding/family combination that it
does not know (e.g. if the document designer says
\fontfamily{ptm}\selectfont) then LaTeX attempts to load the
appropriate .fd file. ``Not known'' means: there was no
\DeclareFontFamily declaration issued for this encoding/family
combination.
The declarations in the .fd file are responsible for telling
LaTeX how to load fonts for that encoding/family combination. If
the .fd file could not be found, a warning is issued and font
substitutions are made.