What is the difference between a mutable and immutable?

Mutable and immutable are English words meaning "can change" and "cannot change" respectively. The meaning of the words is the same in the IT context; i.e.

• a mutable string can be changed, and
• an immutable string cannot be changed.

The meanings of these words are the same in .NET as in other programming languages / environments, though (obviously) the names of the types may differ, as may other details.