Containers are structures and tools for allowing Linux to execute processes in completely separate areas, that is: Virtual Environments (VE).

The separation should be concern the three following:

1. File System (chroot)
2. Process (namespaces)
3. Resource (cgroups)

See also:

http://www.haifux.org/lectures/299/netLec7.pdf

NameSpaces - Processes

Currently, in Linux there are 6 namespaces:

Internal Auditing Practice