Restatements are highly regarded distillations of common law. They are prepared by the American Law Institute (ALI), a prestigious organization comprising judges, professors, and lawyers. The ALI's aim is to distil the "black letter law" from cases to indicate trends in common law, and occasionally to recommend what a rule of law should be. In essence, they restate existing common law into a series of principles or rules.
Restatements cover broad topics, such as Contracts or Property. They are organized into chapters, titles, and sections. Sections contain a concisely stated rule of law, comments to clarify the rule, hypothetical examples, explanation of purpose, as well as exceptions to the rule.
Restatements are not primary law. Due to the prestige of the ALI and its painstaking drafting process, however, they are considered persuasive authority by many courts. The most heavily cited Restatements are the Restatement of Torts and the Restatement of Contracts.
The ALI web site contains information regarding Restatement projects, ALI membership, history and institutional processes.
(http://guides.library.harvard.edu/content.php?pid=103327&sid=1036651, Meg Kribble, mkribble@ law.harvard.edu)
Some older Restatements have been entirely superseded by later editions. However, it is more common for particular annotations on a topic to be superseded by annotations in later editions than for the entire volume to be superseded. It is important to look at all available Restatements on your topic and update the annotations you find useful, to avoid using outdated law or missing key information.
(http://tarltonguides.law.utexas.edu/content.php?pid=95103&sid=721924, firstname.lastname@example.org, and Kasia Solon Cristobal, email@example.com)