QLib is distributed as free software. The word "free" does not refer to price; it refers to freedom: You may run the program, for any purpose, study how it works, adapt it to your needs, redistribute copies and improve the program.

The license under which QLib is distributed is one of the copyleft licenses - using copyright law to remove restrictions on distributing copies and modified versions of a work for others and requiring that the same freedoms be preserved in the modified versions.

Specifically, QLib is released under the LPGL license (full text), a more permissive version of the GPL (full text). The GPL (and LGPL) adds the requirement that you release your improvements of QLib back to the public, so that the whole community benefits.

In other words - you're getting it for free, dependent on your agreement to keep it free and share your enhancements, just as they shared their enhancements with you.

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For non-code part of QLib (documentation, algorithms, etc), the licenseee is basically equivalent, and known as Creative Commons. More specifically Creative Commons Attribution Share-Alike. See there for details.