TeX user groups are organizations for people interested in TeX and LaTeX. They provide support for their members, but also for TeX and LaTeX users in general.
That's a big institution. It was founded in 1980 and has always had a great influence on the development and popularity of TeX. Their homepage is a portal to the TeX world with links to support, documentation, and software.
The LaTeX3 project team maintains the LaTeX 2e standard and develops the next version of LaTeX, called LaTeX3. This website informs about their work and about LaTeX in general.
This user group supports and promotes TeX in the UK. Also this site is a good starting point to explore TeX resources but specifically those offered by the UK TUG.