A TXT record is a DNS record used to hold text information. There is no real limit on what text you may store within a TXT record.
As a TXT record has a hostname, you may use it to assign free text to a specific hostname or zone.
Sender Policy Framework (SPF) data
The most common uses of a TXT record is to store Sender Policy Framework (SPF) data. SPF is used to define the IP addresses and servers from which email messages from your domain are sent. Its' primary use is to combat spam as it prevents against emails being falsified as being sent from you.
Most mail servers - including ours - check incoming messages against the originating domain's SPF data. If the message does not originate from an IP address or server defined within the SPF data, it is considered to be spam.
Other Uses
TXT records may also be used for verification for Google Apps or