American Law Reports (ALR) is a multi-series publication comprised of articles that focus on unsettled areas of the law (often a split between federal circuits or state courts). Each article discusses the issue in detail and provides extensive footnotes to leading judicial opinions. If you find an ALR citation on the issue you are researching, all you have to do is update the key cases -- by running them through Shepard's on Lexis or KeyCite on Westlaw -- and your case law research is pretty much done.
You can search for ALR articles on Westlaw (ALR). Lexis also has all the ALR articles (2NDARY;LEDALR) except for the ALR 1st series. If you have a complete cite, you can pull an article Westlaw West Find&Print using the sample format: "xx alr2d xxxx" or Lexis Get & Print using the format "xx alr 2d xxx".
The print ALR volumes can be found in larger law libraries.
Some ALR articles, called "e-annotations," are available only online. E-annotations may be advance publications that will eventually be published in print. Others will remain electronic-only (e.g., because they are to large to publish in a volume). In either case, they are referenced in the print index.
For a more complete discussion of the ALR series, see Jacobstein and Mersky's Fundamentals of Legal Research.