"Beta phase" generally begins when the software is feature complete but likely to contain a number of known or unknown bugs.
Software in the beta phase will generally have many more bugs in it than completed software, as well as speed/performance issues and may still cause crashes or data loss.
The focus of beta testing is reducing impacts to users, often incorporating usability testing. The process of delivering a beta version to the users is called beta release and this is typically the first time that the software is available outside of the organization that developed it.
Beta version software is often useful for demonstrations and previews within an organization and to prospective customers. Some developers refer to this stage as a preview, preview release, prototype, technical preview / technology preview or early access.
Some software is kept in perpetual beta, where new features and functionality are continually added to the software without establishing a firm "final" release.
Beta testers are people who actively report issues of beta software.
They are usually customers or representatives of prospective customers of the organization that develops the software.
Beta testers tend to volunteer their services free of charge but often receive versions of the product they test, discounts on the release version, or other incentives.