With the LGPL, you can keep your own code closed-source, but you must dynamically link the QT library. That means that your program must require QT's DLL file to run. If your program still works without QT's DLL file present on your system, you've statically linked it, which you cannot do with the LGPL.

The rationale behind it is that your end-users should be able to use a different QT DLL instead -- e.g. if they wanted to recompile QT themselves and use their compiled version with your program.

(IANAL)