As a consumer of software, as opposed to a developer or distributor, this is how I understand the term beta. It has to do with how much you can rely on the software to do what you got it for and how much you can rely that it will not cause damage.
Recreational and even academic software fitness is less important than fitness of the software used to run medical equipment. The area I’m addressing is word processing and spreadsheet and file management packages.
This picture shows the software development cycle for Debian a package that when installed allows you to use your pc for everything you bought it for.
http://en.wikipedia.org/wiki/File:Debian-package-cycle.png
You don’t see the term beta in there, but for software distributors with high integrity, it would be about where the testing and frozen boxes are. One step from the final form. Less scrupulous distributors will sloppily call a program that runs without crashing or a program that compiles with non critical errors a beta. You have only the integrity of the distributor to rely on what stage of development a package is in and how much you can rely on it for what you want it for.
In order to minimize legal liability even legitimate software distributors will include a disclaimer that looks like this
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Literally it says, “ you may have heard that this spreadsheet can add numbers and draw charts without errors, but it wasn’t us that said that and it might not do it properly.”
This is even for the final form of the software. When it is called a Beta, you may find it even less reliable. Again, you are depending on the integrity of the software distributor. A honest, but scrupulous distributor will call a package beta even though it is complete and reliable because he merely wants to minimize leagal liability.