Qt is available under two different licensing schemes—you can choose between a commercial license and an open source one. We will discuss both here to make it easier for you to choose. If you have any doubts regarding whether a particular licensing scheme applies to your use case, better consult a professional lawyer.
When choosing the open source edition, we have to decide between GPL 3.0 and LGPL 2.1 or 3. Since LGPL is more liberal, in this chapter we will focus on it. Choosing LGPL allows you to use Qt to implement systems that are either open source or closed source—you don't have to reveal the sources of your application to anyone if you don't want to.
However, there are a number of restrictions you need to be aware of:
Any modifications that you make to Qt itself need to be made public, for example, by distributing source code patches alongside your application binary.
LGPL requires that users of your application must be able to replace Qt libraries that you provide them with other libraries with the same functionality (for example, a different version of Qt). This usually means that you have to dynamically link your application against Qt so that the user can simply replace Qt libraries with his own. You should be aware that such substitutions can decrease the security of your system, thus, if you need it to be very secure, open source might not be the option for you.
LGPL is incompatible with a number of licenses, especially proprietary ones, so it is possible that you won't be able to use Qt with some commercial components.
The open source edition of Qt can be downloaded directly from http://www.qt.io.
All these restrictions are lifted if you decide to buy a commercial license for Qt. This allows you to keep the entire source code a secret, including any changes you may want to incorporate in Qt. You can freely link your application statically against Qt, which means fewer dependencies, a smaller deployment bundle size, and a faster startup. It also increases the security of your application, as end users cannot inject their own code into the application by replacing a dynamically loaded library with their own.
To buy a commercial license, go to http://qt.io/buy.