The GNU Lesser General Public License is a free, open source license for software and other kinds of works. It is a middle ground between copyleft licenses such as the GPL and permissive licenses like BSD and MIT. The LGPL requires modifications to the work to also be distributed under OSS terms, but allows the work to be included (unmodified) in non-OSS combined works without propagating its terms onto the combined work itself.
As such, the LGPL is considered a more "commercial friendly" license than the GPL, since LGPL libraries can be freely utilized by commercial software.
See the Wikipedia article on the LGPL for more information.
GNU Lesser General Public License v3
Version 3 of the LGPL is the latest version, written in 2007. It is the recommended version of the LGPL to use for new projects.
The LGPLv3 license text can be found at:
GNU Lesser General Public License v2.1
Version 2.1 of the LGPL is an older version from 1999. It is recommended to use LGPLv3 instead if possible.
The LGPLv2 license text can be found at: