Does this mean I have to free the source for my web app?
No*
or is just necessary to release the source for the plug in?
If you distribute the application, then yes, you have to also distribute the source code for the LGPL portion of it.
You would also need to make sure that anyone receiving your application would have a chance of modifying it. This is important for compiled applications - you would need to include the necessary object files so your app can be re-linked with a modified version of the LGPL code. Or, use dynamic linking. The idea is that people need to be able to modify the portion of your code that is LGPL and re-combine it with your app.
Here are some of the requirements of the LGPL in my words (I am not a lawyer):
- There must be some sort of clear separation between the LGPL code and the other code. In particular, it must be possible for the recipient to modify the LGPL code or even completely replace it with other code, such as a modified version or later version of the libary. Therefore, if it is a compiled program, then the LGPL code must either be dynamically linked (like, a separate DLL or shared file) such that it could easily be substituted for a similar library and still be interoperable; or, if it is statically linked, the minimum required source files and/or object files must be provided, to allow recompiling with an alternative library. The non-LGPL portion may not contain any part of the LGPL code except for very simple header files.
- It must be clearly pointed out which part of the code is LGPL covered, including its original copyright notice and the text of the LGPL (including the GPL on which it is based).
- If the combined software displays copyright notices during the course of running, then the copyright notice for the LGPL covered portion must also appear here, along with a link to the LGPL and GPL.
- In some cases, you would need to provide installation information detailing how to use a modified version of the LGPLd code in the combined application.
Those were just some restrictions, but the LGPL also offers you a number of freedoms, that other licenses like GPL don't. If you're distributing a combined work, which includes someone else's LGPL licensed code:
- You don’t need to release source code for the rest of your application (ie, the non-LGPL part). The only exception to this being as described above - if it’s all statically linked then you’d need to provide just enough code (and/or object files) to be able re-link it with an alternative or modified version of the LGPL code. If you are linking dynamically and interacting over a normal API, you don’t need to worry about that.
- You don’t need to release the rest of your application under the GPL. You can use any license you want, including more restrictive proprietary licenses, provided that when you do distribute it you follow the rules.
- Unlike GPL version 3, which prohibits using the code if you are implementing copy protection or DRM software, you can use LGPL version 3 licensed code in an application which includes copy protection or DRM.
I wrote an analysis of the LGPL license recently - you could read this for more information.
Also, please read the actual text of the LGPL. It's not too long.
*you mentioned that the library you're using has a GPL data source. Presumably that means that you won't actually be distributing this GPL data with it, just the LGPL library that uses it.
