ICQ, short form for "I Seek You" when pronounced aloud, has been largely credited for popularizing the sending of "instant messages." When Mirabilis, a previously unknown Israeli company, released ICQ to the Internet it took off like wildfire. ICQ enabled users to send short (<400 character) messages, URLs, and files to each other as well as enabling online chat directly between users. Many other companies subsequently copied the idea; America Online's Instant Messenger (AIM) became quite popular, while most other ICQ clones have languished in relatively obscurity; Microsoft's MSN Messenger and Yahoo! Messenger each maintain a small userbase, while the dozens of lesser-branded ICQ knock-offs have all but disappeared or languished in ignominy.Most instant messaging clients haven't implemented too much in the way of security; it's generally pretty easy to impersonate someone else, change their password, or make someone believe that someone else is sending them messages or files. Instant messaging is interesting because it encourages non-anonymity - users are encouraged to type in their real name, address, web page, interests, picture, and even phone number. This information is shared with other users of the system so, if you are feeling like chatting with a 25-year old male from Sao Paolo, Brazil who speaks Portuguese, you can find a few users who meet that description and begin chatting with any of them.