These contain features that have been well tested (via prior alpha and beta releases) and come with an installer that automatically upgrades and installs everything (and so can be installed by even non-technical people). There are typically one or two of these releases every year.
They have 2 digits in their version number e.g. "1.1" or "2.0".
These are the final step before a general release. The software is stable and well tested and, barring any critical problems, will eventually be re-released as a general release.
They have 3 parts to their version number e.g. "1.1.rc1" or "2.0.rc2".
These contain features that have been tested (via prior alpha releases) and are reasonably stable. They are released as ZIP files so you must know how to unpack them using WinZip or PKZIP. You may also be required to run additional programs (e.g. to upgrade the database, etc.) or copy certain files around.
They have 3 digits in their version number e.g. "1.1.3" or "2.1.1".
These are bleeding edge releases and are made available for people who want to experiment with the latest features as they are being added. These features may not be complete, will contain bugs and are not intended for general use.
Alpha releases have an expiry date but a new release will always be made available before this date.
You should only install an alpha release if you are moderately technical and are aware that the software may have problems.
Alpha releases have 4 parts to their version number e.g. "1.1.3.alpha2" or "2.1.3.alpha1".