By default, Eclipse will run its incremental builders on each file save. The build system for Android is quite complex and not fully incremental at this time.

We have received developer feedback that projects with a lot of resources have started to suffer from much longer compilations. One impact is the possibility of getting a modal build dialog if 2 file saves are triggered in quick succession. This is due to the 2nd build (triggered by the 2nd file save) waiting for the first build to finish. Eclipse requires this to be user-blocking.
With this new option enabled, automatic builds on file save will only generate the R class and compile the Java source code. The bytecode conversion and resource packaging steps are skipped. This should improve automatic incremental build times and reduce the occurrence of modal build dialog showing up.

The apk is then only generated when doing an run/debug launch or an export. For larger projects, this could delay deploying applications to device during debug as the final steps of the build process are executed.

This option is disabled by default. You can enable it in the preferences under Android > Build.

This is the first of several planned changes targeted at making the build process faster both in ADT and the Ant build systems.