The GNU Lesser General Public License (formerly the GNU Library General Public License) is an FSF approved free software license designed as a compromise between the GNU General Public License and simple permissive licenses such as the BSD license and the MIT License. The LPGL was written in 1991 (and updated in 1999) by Richard Stallman and Eben Moglen.
The main difference between the GPL and the LGPL is that the latter can be linked to a non-(L)GPLed program, which may be free software or proprietary software.
The LGPL places copyleft restrictions on the program itself but does not apply these restrictions to other software that merely links with the program. There are, however, certain other restrictions on this software. Essentially, it must be possible for the software to be linked with a newer version of the LGPL-covered program. The most commonly used method for doing so is to use “a suitable shared library mechanism for linking”. Alternatively, static linking is allowed if either source code or linkable object files are provided.
Csound, the music synthesis program, is now completely licensed under the GNU LGPL, which makes it a most welcome part of the open source community.