It is a FSF license, written by Richard Stallman (with legal advice from Eben Moglen, of course.) It has been revised twice. The need for this "Lesser" license was apparent immediately from the start....
In order to support the use of any "proprietary" software at all, a license granting the ability to call library functions (i.e., "linking") has been needed. For example, there are thousands and thousands of programs written in "C" and "C++" which call functions defined and actually performed within the library "glibc". For various reasons, usually but not always having to do with money, some of these Software Authors require that their Source code remain private. (Oracle on Linux is an example of this.)
To allow lots of for-profit companies and coders to use QT, they need the same right to keep their source code private-- even though it links to QT functions, and QT is "Open Source". Formerly, every such Software Developer had to get a separate and unique license from the TrollTech company-- a big hassle, as well as a big expense for everybody involved.
But now, with the pretty-much-standard LPGL grant of license to use, all of these Developers don't have to mess around with "commercial" contracts, or worry if the price will change next year: It will always be free to use, in thses "function calling/object inheritance" ways, from "proprietary" code-- without "forcing" that proprietary code to be made public, as the GPL would.
As Mr. Nystrom said, this change makes use of qt platform much more attractive, it's a very big deal. I'm very grateful to Nokia/Trolltech for this change-- they're giving up a pretty big stack of "commercial contract" money uncollected by providing their wonderful library of cross-platform, cross-language Widgets for free.
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