"CPU units" is just a CPU priority parameter. A VPS-Basic has a priority of 200, a VPS-Standard has a priority of 400, and a VPS-Enhanced has a priority of 800.

As a simplified example, let's say we were running only one VPS-Basic and one VPS-Enhanced on a server. If both VPSes had processes using 100% of the CPU, the VPS-Basic would get 20% of the CPU and the VPS-Enhanced would get 80% of the CPU (200 + 800 = 1000, 200/1000=0.20, 800/1000=0.80). But, not all VPSes demand 100% of the CPU 100% of the time. So, a VPS-Basic could still get 100% of the CPU if no other VPS accounts on the hardware node needed it, but that percentage would drop down as soon as another VPS needed CPU time.

For more in-depth information, you can read through the following thread on our forums:

http://forum.schmolie.com/index.php?act=ST&f=21&t=415