The aims of the project are to perpetuate and develop the spirit and philosophy of TeX, whilst respecting Prof. Donald Erwin Knuth's wish that TeX should remain frozen. e-TeX is the first concrete result which can be used in two different modes: in compatibility mode it is supposed to be completely interchangable with standard TeX. In extended mode several new primitives are added that facilitate (among other things) bidirectional typesetting.
The formats are often prefixed with an `e', hence eTeX as the extended version of TeX and eLaTeX as the extended version of LateX. However, eplain is an exception to this rule.