User blog comment:Duckcall00/CoD Black Ops Beta/@comment-789991-20100820164645/@comment-789991-20100820170344

There are two types of beta; public and internal. The betas you've played are public ones. They're cleaned up for the most part, but might have game balance issues or need to be optimized more. They're the ones you can access via codes and the like.

Internal ones are much less stable, have a ton of WIP content (such as untextured levels) and leftovers from scrapped content, and generally aren't meant to be seen by anyone outside of the developers. The only way people get access to them is via leaks from the developers.

Here's an example of an internal beta that was leaked: http://half-life.wikia.com/wiki/Half-Life_2_Beta