netfilter/iptables is - like all of the Linux Kernel - Free Software (sometimes referred to as Open Source Software), distributed under the terms of the GNU GPLv2 only. 
Please, note that some source code files might differ, and in that case it is explicitely stated in the header of every file.

The GPL also contains some obligations. If you distribute netfilter/iptables code in binary form, you have to offer the source code, too.

The netfilter/iptables project has made available some more detailed information on this subject:

The remainder of this page as pdf (An Article on the licensing of netfilter/iptables by Harald Welte

An Example disclaimer to be used as an appendix in a product manual.

The TeX source code of those documents can be found via our ViewCVS interface.

Contact the coreteam in case you have any further questions.