CTAN is an acronym for Comprehensive TeX Archive Network. It is a collection of tools to transfer LaTeX into eBooks.
TeX4ht does not parse (La)TeX source (so that it avoids the difficulties encountered by many other converters, arising from the irregularity of (La)TeX syntax). Instead, TeX4ht uses (La)TeX itself (with an extra macro package) to produce a non-standard DVI file that it can then process. This technique allows TeX4ht to approach the robustness characteristic of restricted-syntax systems such as hyperlatex and gellmu.