The (L)GPL requires "derivative works" to be licensed under the same terms as the original. The LGPL contains an exemption for works which reference the original as a dynamic library (.dll, .so, .dylib, etc.), such that (unlike with the GPL) non-LGPL software can use LGPL libraries. In other words: if your program only dynamically links against Qt, then it is not a "derivative work" and therefore doesn't have to be licensed under Qt's own license terms.