What does this mean: v=spf1 include:_spf.google.com ~all?

In addition to my previous question, what does this DNS entry mean:

 v=spf1 include:_spf.google.com ~all  ?

This is a SPF record.

The include directive includes SPF records for the domain _spf.google.com. This way Google can add and remove permitted mail servers without you having to change your DNS record whenever Google updates their settings.

The ~all means that receiving mailservers should accept Emails even the SPF verification failed, but mark them as failed. This is called "SoftFail". In comparison -all is a "HardFail", saying "don't accept the Email if the SPF verification failed.

When you are configuring email handling for your domain, you might need to know the IP addresses of the Google Apps mail servers, to help prevent Google messages from being marked as spam. (...)

v=spf1 include:_spf.google.com ~all

(...) With this method, your domain automatically inherits changes to the Google IP addresses as they happen.

http://support.google.com/a/bin/answer.py?hl=en&answer=60764