OFBiz comes in three flavors, depending on your tolerance for change.
If you don't know what to choose, we recommend Apache OFBiz Getting Started
Trunk
The "trunk" is the actively developed version of OFBiz. New features are added here, and this is where new bugs are reported and first fixed.
Releases
Releases are branches of the code made every year or so. These branches are not actively developed. Bugs that appear in branches may be fixed, usually by someone back-porting the fix from trunk code, and sharing the patch with the rest of the community. Releases are more stable than the trunk, and may be somewhat easier for new users to deal with.
Release 10.04
This is the most recent release of OFBiz, made in April 2010.
Release 9.04
This is the previous release of OFBiz, made in April 2009.
Release 4.0
This was an older release, and the last release before 9.04. It is not recommended for new installations, and is only maintained for supporting already existing installations.