It depends...
It depends on the libraries you used and their license. Most libraries have a quite permissive license, so in general, just giving credit, eg. in your site, is enough.
But if you use a GPL library, without classpath exception, then you are forced to release your source code...
Such GPL libraries are quite rare, though, so perhaps it won't apply to your case.

Agree with PhiLho 100%; it depends entirely on the license.  Some of the common licenses are:

MIT
BSD
GPL
Creative Commons (less common for software)

The MIT and BSD Licenses are 'permissive' licenses, meaning that you get the ability to reuse, remix, redistribute, etc. for commercial OR non-commercial purposes.  The licenses themselves are actually quite easy to read (not too much legal speak); look them up on Wikipedia to get the exact terms.  But the basic idea is that you can do whatever you want with the software as long as you:

1) credit the author
2) if you redistribute the code it must be redistributed with the license.

However, these licenses allow you to incorporate material under these licenses into proprietary software, which can then be released closed-source.

The GPL (General Public License) is less permissive but more 'free'.  The GPL gives you the same rights to reuse, remix, redistribute, etc. as the BSD/MIT licenses, but with an additional copyleft clause.  Copyleft means that any derived works may only be redistributed with the exact same license.  This means that even if you modify a piece of software released under the GNU GPL, you may not re-release it as a piece of closed-source software.

Then there's Creative Commons licensing, which is meant as a general license (not just for software), but is starting to gain traction for software use.  They have six licenses, so you need to identify which license the work in question is using and look it up.  However, like the BSD and MIT Licenses, these licenses have an easy-to-read version in plain old English, so its not difficult to understand the terms of the license!

Find out more about creative commons licenses <a href='https://creativecommons.org/licenses/'>here</a>.

In summary:
-All of these licenses require that you credit the author
-All of these licenses require that if you redistribute the software, it is redistributed with the license
-GPL requires that you release the source of derivative works
-Creative Commons has a variety of clauses regarding derivative works, commercial use, and code release.  Check the specific license for more information.

I hope that was helpful!