What does the CAP Theorem actually say?

The CAP Theorem says that it is impossible to build an implementation of read-write storage/system in an asynchronous network that satisfies all of the following three properties:

• Availability – will a request made to the data store always eventually complete?
• Consistency – will all executions of reads and writes seen by all nodes be atomic or linearizably consistent?
• Partition tolerance – the network is allowed to drop any messages.

More informally, the CAP theorem tells us that we can’t build a database/system that both responds to every request and returns the results that you would expect every time.