If you want to distribute a combined work, you’ll have to use the following license;
Proprietary Source code + GPL Source code
- Either static or dynamically linked: You must release both parts as GPL.
Proprietary Source code + LGPL Source code
- statically linked:
- Either you must release both parts as LGPL.
- Or provide everything that allow the user to relink the application with a different version of the LGPL source code. In this case the other requirements are the same as if it was dynamically linked.
- dynamically linked: LGPL code stays LGPL, you can keep the proprietary code proprietary.
See also executing a (L)GPL program from proprietary Source code.
Update (November 2014): A Comprehensive Tutorial and Guide contains a clear an detailed description of the (L)GPL and its usage, including distribution. I recommend it for more details.