Definitely Google and check the JavaDocs for the JSTL taglibs.
But basically, here's how it works. A tag library provides a set of custom tags that perform some type of function. I just finished a blog about using some basic
JSF tags in a simple web page.
(My JSF Blog) Tags in a JSF page look like this:
Here you have an outputLink tag, an outputText tag and view tag. Now, those look like HTML/XML type elements, but they provide some logic and functionality, and that functionality is written in
Java. So, while the developer who uses the tag is shielded from the implementation, the fact is, there is definitely a Java class somewhere that provides the implementation of those tags. So, yes, the code you saw in the JSTL library was probably the implementation of a particular tag. However, that's not where you want to go to find out about how to use the tag. As a web developer, you just want to focus on the tag.
-Cameron McKenzie