Ispell, is one of a family of programs used to compare a ``dictionary'' to a collection of words (your text), to build a dictionary, and other related operations. Some systems may have a program called spell which may be a shell script which invokes ispell with special options.
Ispell is not limited to working with plain text files. It can be used with TeX, LaTeX, nroff and troff files, without the special formatting commands getting flagged. And it is reasonably intelligent about prefixes and suffixes.