Struts 2 is the newest version, however there are many projects that use Struts 1 yet.
Struts 2 is quite different than Struts 1 (actually, Struts 2 was a project called Webwork and joined to Apache team and changed its name to Struts 2).
Unfortunately Struts 2 official documentation is not so good, however there are many resources in the internet. A good site to check out is that:
http://www.roseindia.net/struts/struts2/index.shtml
Regards, Jair Rillo Junior
http://www.jairrillo.com/blog, SCJA 1.0, SCJP 1.4, SCWCD 1.4, SCBCD 5.0, IBM SOA Associate (Test 664).