The -BETA postfix was used a long time ago to mark versions as beta. I have dropped this convention, since versions now flow from beta to production.
At the time of writing, on Google Play:
0.9.21 is in production.
0.9.24 is in beta.
0.9.26 is for a select few people who are running some Motorola devices and want CD-rom support, but it is still somewhat buggy. It'll come to beta eventually.