Westlaw is a licensed database with federal and state case law with extensive coverage.
You generally want to turn to Westlaw for case law after you have already surveyed secondary sources on your topic and after using free online sources for case law such as Google Scholar and/or used print digests.
There are four main ways to find case law:
Terms and connectors:
To do terms and connectors searching, after selecting a case law subset of Westlaw, click on the "advanced" link next to the main search box. On the right side of the screen, you will see reminders of the different symbols you can use. On the left, you will see the different field search options you have. (There are by far more field search options for case law searching than for other resources like statutes.) To see what these fields actually search, view the PDF Westlaw has provided indicating how it indexes individual cases.
One of the most effective ways to use fields is to search within the editorial synopses (sy), containing introductory summaries of each case's facts and holdings, and the digests (di), made up of headnotes (he) and topics (to). These fields may be searched together using the commands directly [ Ex: SY,DI(employ! /1 discriminat!) ] or by selecting the special Synopsis/Digest field.
By subject--West Key Number System:
The West Key Number System is a subject arrangement of the law into about 400 topics. (The print equivalent is a West digest.) Key numbers are assigned to each headnote within an individual case that a West editor has flagged as an important point of law. You can get directly into the West Key Number System under "Tools" or, after selecting "Cases," you will see it on the right under "Tools & Resources."
One oddity about Westlaw is that there is no online equivalent to West's print digests' index (Descriptive Word Index) used to get into West's subject arrangement of cases. Instead, you can browse the over 400 topics, but it tends to be difficult to know which topic to choose even though they come with scope notes. You can also do a natural language search of the Key Number System; "Title Search" on the right is more narrowly focused than using the main search box at the top. Perhaps the best way to get into the Key Number System though is to do an initial search and build from the ones listed in the individual cases that you find on point.
However you get into the West Key Number System, clicking on a topic or keyword number will bring up a search screen that allows you to specify jurisdiction and add keyword search terms.
Display of case law search results:
When viewing search results, on the left are various ways to search within or filter your results (most notably, jurisdiction and date); on the right are displayed "Related Documents," including secondary sources such as legal encyclopedias and treatises.
Westlaw is more comprehensive and sophisticated than free online resources. Westlaw is still unique in its deployment of the key number system for subject searching.
This is an expensive resource. You should be aware of how your firm or employer is charged for its use. Using it cost effectively within that framework requires thought and training on how best to use the database.