Who/what is behind LaTeX?

Citing the CTAN LaTeX entry:

LaTeX is a widely-used macro pack­age for TeX, pro­vid­ing many ba­sic doc­u­ment for­mat­ing com­mands ex­tended by a wide range of pack­ages. It is a de­vel­op­ment of Les­lie Lam­port's LaTeX 2.09, and su­per­seded the older sys­tem in June 1994. The ba­sic dis­tri­bu­tion is cat­a­logued sep­a­rately, at la­tex-base; apart from a large set of con­tributed pack­ages and third-party doc­u­men­ta­tion (else­where on the archive), the dis­tri­bu­tion in­cludes:

– a bunch of re­quired pack­ages, which LaTeX au­thors are “en­ti­tled to as­sume” will be present on any sys­tem run­ning LaTeX; and – a min­i­mal set of doc­u­men­ta­tion de­tail­ing dif­fer­ences from the ‘old’ ver­sion of LaTeX in the ar­eas of user com­mands, font se­lec­tion and con­trol, class and pack­age writ­ing, font en­cod­ings, con­fig­u­ra­tion op­tions and mod­i­fi­ca­tion of LaTeX.

It's published under the LaTeX Project License 1.3 and is considered to be free and changeable. (See LaTeX Project too)

It was developed initially by Leslie Lamport to ease the usage of TeX (which was designed by Donald E. Knuth also named The Grand Wizard (not to be confused with the name from the infamous and dreadful KKK) in the late 70s/early 80s)

Although there are commercial TeX versions which have to be paid for the majority of users download LaTeX (and XeLaTeX/LuaLaTeX as well as TeX) either as the TeXLive or MikTeX distributions or directly from the CTAN server.

A list of current maintainers of LaTeX2e and LaTeX 3 core can be found here: LaTeX 3. A lot of other people design(ed) additions as style oder class packages, all based on the core -- everybody is allowed to use LaTeX and to provide own additions and formats -- that's one of the most thrilling (and sometimes difficult) features of this 'programming' language.

LaTeX users and developers meet in conferences and are organized in global communities such as TUG (TeX User Group), Dante (a German/Austrian TeX user group) and several other local/national groups as well as online like here on TeX.SX, goLaTeX, LaTeX community and some others.

I would also mention Heiko Oberdiek if only because the incredible Hyperref is really what is getting LaTeX on the Web via pdf. And also whoever MathJax is because it is only because of MathJax that LaTeX mathematics is getting on the Web.