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A message digest (sometimes called a hash) is a small fixed length "summary" derived from a longer piece of data. For a cryptographically useful digest, any change to the data results in a different summary and it is effectively impossible to generate a block of data to match a particular digest. You can ensure that a message you have received has not been tampered with if you can calculate the message's digest and compare it with one created before transmission.
TLS uses the MD5 and SHA-1 digest algorithms. MD5 produces a 128 bit result, SHA produces a 160 bit result. Many applications are now moving to SHA-1 though MD5 is still in widespread use.