The Workbench periodically saves a snapshot in order to reduce the risk of losing data due to crashes. The default period between saves is 5 minutes, but you can change this from the General > Workspace preferences page. From that page you can also set Eclipse to save all modified resources before a manual build.
Previously created projects and saved editor data are never lost as they are written to disk immediately upon save. However, there is a risk of data loss in these areas:
Developers of plug-ins can also choose to participate in the autosave lifecycle and have the Workbench also save the state of their plug-ins. See "Workspace save participation" in the Platform Plug-in Development Guide for details.
After a crash you may find that Eclipse will not open because it has (erroneously) detected that your workspace is in use. To remedy this, enter:
rm workspace/.metadata/.lock
You can now restart Eclipse.