Statutes are primary authority, mandatory authority in their jurisdiction, and enacted by legislatures.
Statutes are first enacted by legislatures and collected as "Session Laws". This is a generic term; some states collect them in sets called acts, public acts, laws, etc.
Session laws are organized as they were passed, chronologically. Session laws are re-arranged by subject into "Codes"- these are the statutes you use on a daily basis.
To be included in the Code, a law must usually:
The classic example of acts which you'll find in the session laws but not in the code are budgetary provisions. Budget measure, while certainly affecting the public welfare, are only good for the year they were passed.
Most states publish their own Official codes, or statutes. West (and other publishers) frequently publish unofficial statutes. Generally, the unofficial statutes are published faster and generally include more material (annotations) which is helpful to the researcher.
As an example- Oklahoma completely publishes their statutes once every ten years (1991, 2001, 2011, etc.). In the interim they publish supplement volumes detailing only things that have changed - so that if you need to find a statute you potentially have to check (for example) the 2001 statutes, then the 2002 supplement, 2003, etc. The Annotated statutes for Oklahoma, published by West, are cumulative- that is you only have to check the main volume, pocket part, and any later session laws from your current session.
Most statute sets, even if unannotated, contain at least some "extras"; for example the Oklahoma Statutes contain a few history notes which will tell you when the statute was enacted and citations to prior session laws which amended it. Annotated sets though contain much more, such as:
There are two reasons you need to update statutes:
To perform either task in print, first check the pocket part. Note that if the statute is listed in the pocket part it might be because either the statute was amended or there have been newer cases interpreting it, or both.
After checking the pocket part, check the end of the statute set to see if there are any cumulative pamphlets, then check the later session laws (if the legislature is in session).
Washburn University School of Law maintains an excellent page of state statue (and other) resources at http://www.washlaw.edu/uslaw/index.html