Eclipse is an IDE (Integrated Development Environment) with editors, debuggers, and panels to view type hierarchies and show all uses of particular classes and variables. It is an excellent tool to edit and debug software, including J2EE applications. With Eclipse you can check out the current version of source from Subversion, make changes to the code, test the changes, and check the modifications back into Subversion.
Maven is a batch tool for building and packaging production artifacts. An artifact may be the WAR file for a Web application, or the EAR file for a J2EE application (containing WARS and EJBs), or it may just be a component library packaged as a JAR file. In order to create the artifact, Maven will compile any source, copy data files, and even download over the internet any referenced public Java JAR libraries. Maven "pulls" all these pieces together to create the production deploy file.
Eclipse is not good at building artifacts, and Maven has no editors or debuggers. However, if you combine the two, then you have full coverage for the entire application development cycle.
At Yale, Maven is not optional. The software development and deployment conventions mandate the use of Maven to build any Yale application. Eclipse, on the other hand, could be replaced by some other IDE (or a text editor if you really object to the modern world). So any Yale software has to be a Maven project with a pom.xml file and the directory structure mandated by Maven conventions (Java source is in src/main/java and Web files are in src/main/webapp). It is also a Yale convention that projects be stored in Subversion.
If you check out a Subversion directory into an Eclipse project in the workspace, and the directory contains a pom.xml file, then the Eclipse support for Maven (called "M2E") can read the POM file and configure the Eclipse project to approximately duplicate Maven conventions. That is, Eclipse will be told to compile source files from the same directories where Maven expects source, and to put the output class files in the same directory where Maven would have put its class files. M2E will also download JAR libraries listed as dependencies into the Maven repository and will create an Eclipse Build Path that uses these dependency JARs to resolve references during compilation.
However, Maven has a large library of special purpose or extended function "plugin" modules that can be added to do extra processing steps. M2E supports only the most commonly used processing to compile Java and build JAR or WAR files. It does not duplicate the full capability of batch Maven to run preprocessors, generate source files, or perform special packaging steps. For certain projects you may have to run Maven once in batch to generate all the files, then do a little manual configuration of the Eclipse project to add things the standard M2E could not configure.
For example, CAS uses the "WAR Overlay" technique supported by Maven. One Maven project builds a vanilla CAS WAR file with no Yale code. A second project contains all the Yale customizations. The second project is applied as an "update" to the WAR file generated by the first project. Files in the second project with the same name replace files in the first project, while new names add new files.
Eclipse does not understand the "WAR Overlay" type of processing. It sees two projects that each produce a WAR file. It does not understand that the output of the first project will be changed using files created in the second project. So Eclipse produces two WARs, one useless to Yale because it is vanilla code and the other useless because it contains only the Yale modified files but is missing all the unmodified HTML and XML files in the first project. Eclipse cannot build a working CAS application, for that you need to run Maven in batch. However, M2E can configure Eclipse so that all the source files can be edited and the other source files and dependency JARs have been properly linked in so autocomplete and all the other IDE tools work properly.
Every new release of Eclipse changes the set of features that are automatically included and those that have to be manually installed. Furthermore, the Maven integration of M2E gets a bit smarter in each release. Generally this makes things simpler and there are fewer extra steps to get a useful development environment. However, a "cookbook" style list of specific instructions will not be exactly correct in the next release, so these instructions will be a bit more descriptive of the how and why so you can adapt if the exact steps change from year to year.
This cannot be a serious introduction to either Maven as a comprehensive project building utility or Eclipse as an IDE. There are books that cover the intricate details of each. However, it will cover a bit more than the two page tutorial that you find with either system. Move though the material sequentially and skip pages that cover material with which you are already familiar, although the objective here will be to provide more explanation than you normally get about how things work. By design, each major topic has its own URL that you can bookmark or link to.