The Modules system manages a user's shell environment. Modules can be "loaded" and "unloaded" in a clean fashion. Loading a module typically alters or sets shell environment variables such as PATH, LD_LIBRARY_PATH, etc.
Useful module commands include:
module avail # lists available modules
module list # lists currently loaded modules
module help moduleName # help on the module
module display moduleName # displays changes that are made to
# the environment by loading the module
module load moduleName # loads the module
module unload moduleName # unloads the module and removes all
# changes that it made in the environment
module purge # unloads all modules