The current OpenBSD release is 2.9. This is the tree most commonly used by OpenBSD developers. The -current tree contains all the code that is planned for the next release. From time to time, brave souls will abandon the formal releases and run openbsd-current, usually to take advantage of features that have not yet made it into the formal releases. Between formal releases, a series of snapshot releases are made available.
Snapshots are test releases of the -current source tree.
The ports tree has a special policy for following the src tree. Due to changes in the underlying libraries, moving directly from a release to -current is not always easy. Once the snapshot is installed, the latest source can be fetched and built . This procedure should eliminate most toolchain and library problems. The snapshots used to be taken temporarily offline right around the release time of a new version.