IANALIANAL is Usenet and chat abbreviation for "I am not a lawyer". TINLA is an abbreviation for "This is not legal advice".
One or both of these abbreviations usually precede opinions about law warning people not to take their opinion as professional legal advice. Many jurisdictions have legal restrictions on actually, or even appearing to, give legal advice, or otherwise practicing as a lawyer without legal qualifications and official registration.
This expression became more and more common on online web forums starting from the year 2000, when debate about file sharing and P2P applications started to focus on copyright, privacy and fair use issues. Frequently, people start their comments with IANAL to say that they are not a legal professional before stating their opinion.
Sometimes, in a somewhat humorous way, one of the chat participants is actually a lawyer, and will start his message with IAAL (I am a lawyer).
On Slashdot it is popular to start postings with "IANAL, but...".
Note: Most Wikipedians are not lawyers, and Wikipedia does not give legal advice