A font consists of a number of glyphs. In order that the glyphs may be printed, they are encoded, and the encoding is used as an index into tables within the font. For various reasons, Knuth chose deeply eccentric encodings for his Computer Modern family of fonts; in particular, he chose different encodings for different fonts, so that the application using the fonts has to remember which font of the family it's using before selecting a particular glyph.
When TeX version 3 arrived, most of the excuses for the eccentricity of Knuth's encodings went away, and at TUG's Cork meeting, an encoding for a set of 256 glyphs, for use in TeX text, was defined. The intention was that these glyphs should cover 'most' European languages that use Latin alphabets, in the sense of including all accented letters needed. (Knuth's CMR fonts missed things necessary for Icelandic and Polish, for example, but the Cork fonts have them. Even Cork's coverage isn't complete: it misses letters from Romanian, Eastern and Northern Sami, and Welsh, at least. The Cork encoding does contain "NG" glyphs that allows it to support Southern Sami.) LaTeX refers to the Cork encoding as T1, and provides the means to use fonts thus encoded to avoid problems with the interaction of accents and hyphenation (see hyphenation of accented words).
The only Metafont-fonts that conform to the Cork encoding are the EC fonts. They look CM-like, though their metrics differ from CM-font metrics in several areas. The fonts are now regarded as 'stable' (in the same sense that the CM fonts are stable: their metrics are unlikely ever to change). Their serious disadvantages for the casual user are their size (each EC font is roughly twice the size of the corresponding CM font), and there are far more of them than there are CM fonts. The simple number of fonts has acted as a disincentive to the production of Adobe Type 1 versions of the fonts,, but several commercial suppliers offer EC or EC-equivalent fonts in type 1 or TrueType form - see commercial suppliers, and free auto-traced versions are available. What's more, until corresponding fonts for mathematics are produced, the CM fonts must be retained because some mathematical symbols are drawn from text fonts in the CM encodings.
The EC fonts are distributed with a set of 'Text Companion' (TC) fonts that provide glyphs for symbols commonly used in text. The TC fonts are encoded according to the LaTeX TS1 encoding, and are not viewed as 'stable' in the same way as are the EC fonts are.
The Cork encoding is also implemented by virtual fonts provided in the PSNFSS system, for PostScript fonts, and also by the txfonts and pxfonts font packages (see "choice of scalable fonts").
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=ECfonts