You can look up the full [http://jcp.org/en/introduction/glossary Glossary] and the [http://jcp.org/en/introduction/faq FAQ] for full explanations. Below are some simplified terms.
:[http://jcp.org/en/home/index Java Community Process] - the process by which interesting Java-type stuff makes it into the official language, both Java SE/EE and Java ME
:[http://jcp.org/en/jsr/all Java Specification Request] - specifications for bits of functionality to be added to Java. These can be very small things, they can be new versions of existing functionality (e.g. JMS 2.0), they can be large chunks (e.g. Java 7) or even changes to the process itself ([http://jcp.org/en/jsr/summary?id=348 JSR 348]).
:[http://jcp.org/en/participation/committee Executive Committee] - members of the process who vote at various stages in the life of a JSR to decide whether it will progress further. The London Java Community has a seat on the JCP SE/EE EC. Try saying that after too many glasses of wine.
:Expert Group - the group of people for a specific JSR who help to develop that JSR. These guys produce the appropriate documentation and basically try to get the JSR completed.
:Each JSR/EG will have one or more Spec Leads. These guys will be the main point of contact for a JSR and will be responsible for driving it forward.
:Technology Compatibility Kit - required in order for a JSR to be completed. It ensures that an implementation of a JSR complies with the specification. There is some information on creating a TCK on the [http://java.net/projects/ljc-london-jug/pages/JSR-310 JSR 310 page].