CB stands for Cumulonimbus cloud formations, which are very dangerous for all types of aviation from 747's to 152's!! The are associated with extreme weather conditions like hail, and extreme turbulence and winds. They also contain very strong updrafts and downdrafts. Thus they are not good for aircraft and commercial traffic uses the weather radar to navigate round them. The actually life span of the cloud formations can be quite short but they can extend from anything from 1500 feet to 30000 feet, so they get pretty big.
So basically they are not good news!!
Hope this helps
Tom