Eclipse Project Build Types

R - Releases
Releases are builds that have been declared major releases by the development team - for example "R1.0". Releases are the right builds for people who want to be on a stable, tested release, and don't need the latest greatest features and improvements. Release builds always have an "R" at the beginning of the name i.e. R1.0, R2.0 etc. Non-release builds are named according to the date of the build - for example 20060427 is the build from April 27, 2006.

S - Stable Builds
Stable builds are integration builds that have been found to be stable enough for most people to use. They are promoted from integration build to stable build by the architecture team after they have been used for a few days and deemed reasonably stable. The latest stable build is the right build for people who want to stay up to date with what is going on in the latest development stream, and don't mind putting up with a few problems n in order to get the latest greatest features and bug fixes. The latest stable build is the one the development team likes people to be using, because of the valuable and timely feedback.

I - Integration Builds
Periodically, component teams version off their work in what they believe is a stable, consistent state, and they update the build configuration to indicate that the next integration build should take this version of the component. Integration builds are built from these stable component versions that have been specified by each component team as the best version available. Integration builds may be promoted to stable builds after a few days of testing. Integration builds are built whenever new stable component versions are released into the build.

N - Nightly Builds
Nightly builds are produced every night from whatever has been released into the HEAD stream of the CVS repository. They are completely untested and will almost always have major problems. Many will not work at all. These drops are normally only useful to developers actually working on the Eclipse Project.
J,M - Maintenance Builds
Maintenance builds are produced periodically to incorporate fixes into an existing release. They are typically built from tagged plug-in and feature pojects in a maintenance stream of the CVS repository (i.e. R1_0_maintenance). Maintenance builds are promoted to a minor or service release (i.e. R1.1 or R1.0.1) after development teams have deemed one to be stable following one or more test-fix pass cycles.
K,L - Legacy Builds
Legacy builds are maintenance builds for some older release, after the usual service releases (n.n.2 release). Legacy builds do not intend to culminate into any release, and are produced for escalated fixes only on request. Typically, companies requesting a legacy build need to contribute the resources for doing that build themselves.