Based on the software release lifecycle page, and my personal experience, here's how I'd characterize each phase of software development:
The software is still under active development and not feature complete or ready for consumption by anyone other than software developers. There may be milestones during the pre-alpha which deliver specific sets of functionality, and nightly builds for other developers or users who are comfortable living on the absolute bleeding edge.
The software is complete enough for internal testing. This is typically done by people other than the software engineers who wrote it, but still within the same organization or community that developed the software.
The software is complete enough for external testing -- that is, by groups outside the organization or community that developed the software. Beta software is usually feature complete, but may have known limitations or bugs. Betas are either closed (private) and limited to a specific set of users, or they can be open to the general public.
The software is almost ready for final release. No feature development or enhancement of the software is undertaken; tightly scoped bug fixes are the only code you're allowed to write in this phase, and even then only for the most heinous and debilitating of bugs. One of the most experienced software developers I ever worked with characterized the release candidate development phase thusly: "does this bug kill small children?"
The software is finished -- and by finished, we mean there are no show-stopping, little-children-killing bugs in it. That we know of. There are probably numerous lower-prority bugs triaged into the next point release or service pack, as well.