Quick and dirty explanation of GPL/LGPL.

1. You can charge whatever you want for your software.
2. You don't have to show any of your code to the public.
3. When using libraries licenced under the GPL you have to give, anyone you sell your program to, the source code to your program, and your code must be under a GPL licence. So anyone you sell your program to will be free to distribute it to the world for free.
4. When using libraries licenced under the LGPL you don't have to give, the source code to your program, to anyone. You only have to show the source code, for any changes you have made to the libraries them selves, not the code that just uses the libraries. And you can put whatever licence you want on the parts of your program that only uses LGPL libraries.