Decentralized Instant Messengers
Decentralized instant messengers differ from other messengers by putting the user in control of their account - there is no central server where accounts are registered, they are created by each user on that user's own device.
Distributed Instant Messengers
Distributed instant messengers allow a user to create an account on one server, and communicate with users on other servers run by different people or organizations.
Other Instant Messengers
The following instant messengers follow the common model of centralized servers under the control of one organization managing each user's account. These systems generally do not allow exchanging messages with users on other systems.