Surprising question as nothing like a quick search can't find easily:
From the Wikipedia page at https://en.wikipedia.org/wiki/WEB:
WEB is a computer programming system created by Donald E. Knuth as the
first implementation of what he called "literate programming": the
idea that one could create software as works of literature, by
embedding source code inside descriptive text, rather than the reverse
(as is common practice in most programming languages), in an order
that is convenient for exposition to human readers, rather than in the
order demanded by the compiler.
WEB consists of two secondary programs: TANGLE, which produces
compilable Pascal code from the source texts, and WEAVE, which
produces nicely-formatted, printable documentation using TeX.
CWEB is a version of WEB for the C programming language, while noweb
is a separate literate programming tool, which is inspired by WEB (as
reflected in the name) and which is language agnostic.
The most significant programs written in WEB are TeX and Metafont.
Modern TeX distributions use another program Web2C to convert WEB
source to C.
More info in the highly recommended book from the author:
Literate Programming (Center for the Study of Language and Information - Lecture Notes) Paperback – June 1, 1992
ISBN-13: 978-0937073803 ISBN-10: 0937073806
Check the reviews for the book at Amazon.com or better yet, buy the book and start reading.