ZBR

Another term borrowed from Microsoft's project management philosophy, ZBR is where the end really begins.

It means Zero Bug Release, though it doesn't mean shipping a bug-free game...

Beta

Also coming from traditional software development, Beta usually has a less controversial definition than Alpha.

The short version is that a beta contains everything, including the bugs.

However, in games development, Beta has been taking another meaning since the last few years.

Alpha

Borrowed from traditional (i.e. non-games) software development, Alpha designates a key stage in a game's development. The definition of Alpha for traditional software is, according to Wikipedia:

Alpha software can be unstable and could cause crashes or data loss. [...] The alpha phase usually ends with a feature freeze, indicating that no more features will be added to the software. At this time, the software is said to be feature complete.

In game development, the definition is a bit more blurry.