ICQ is an instant messaging application originally developed in 1996 and developed for more than two decades since. It was originally owned by Mirabellis and later acquired by Mail.Ru. As of 2013, the app had 11 million users monthly.
The app's name is not an acronym. It derives from the English phrase, "I Seek You".
Windows, Mac OS X and Linux are officially supported, as well as mobile platforms such as iOS and Android. Other platforms may have recent versions or at least older versions available that are still compatible with its protocol.
Rather than a username, users identify themselves first with a number that is unique to them. To add another user, you must know their number, and for another user to add you, they must know yours. After that, you can see each others' display names and differentiate each other more easily.
It is not possible as of this writing in recent versions to get the older sounds from ICQ or extensively modify the app's notification sounds. On Windows, you can turn off notification sounds by opening a menu available just under your profile. On other platforms, the process is usually similar.
Calls and video chat are all encrypted end-to-end as of this writing. This means that your communications are almost entirely private. Text chat is likely to be encrypted as well.
On mobile platforms, you may convert ICQ chat messages into audio by holding your device to your ear and listening.
ICQ allows group chats with unlimited numbers of people. You can either create a group chat from inside of a chat or from your contact list. On most versions of ICQ, the option to create a group chat is near the bottom of the screen. On desktop versions, it can sometimes be accessed from the top bar while viewing either of these screens.
On some versions of the app, you may be able to find chat rooms directly from the client. This feature is largely deprecated, and whether you will be able to find others' chat rooms to join, outside of those you know, depends largely on what version you are using. Some websites offer links to ICQ chats you can join at any time, so using a search engine can help if you cannot find a chat search feature.
Previously, it was not possible to delete one's account; it could only be disabled. You may now delete your account via ICQ's website. However, publicly visible messages, such as those in group chats, may not be deleted. Consider editing or deleting sensitive information before deleting your account.