Quite a bit of legal information is available online. You can access the text of all U.S. Supreme Court opinions, for example. You can also find most appellate court decisions state and federal after a certain date. But you still cannot find many older lower court decisions online. Also, the vast majority of state and federal trial court opinions are still not available online for free. Most lawyers subscribe to an electronic database service the two biggest are Westlaw and Lexis/Nexis that provides access to nearly all legal authorities.
Annotated codes are important because they contain not only the text of the various laws (in the code) but they also provide annotations that give information about different legal sources that mention or cite different laws. For example, if you look up 42 U.S.C. § 1983 in the United States Code Annotated, you will find pages upon pages of citations to various judicial opinions that cite, mention, or explain the reach of this particular law. In other words, an annotated code allows a legal researcher to find both an applicable statute and perhaps some citations to cases that explain that statute or law.
Statutes are cited differently than cases. Federal statutes or laws consist of three primary elements: (1) a title number; (2) abbreviation for the federal code; and (3) the section number. For example, 42 U.S.C. § 1983 is an important federal civil rights statute; 42 refers to the title (Public Health and Welfare) of the U.S. Code where this statute is found; U.S.C. stands for United States Code (where federal laws are codified or arranged); and 1983 refers to a section number.