Instant messaging has grown exponentially from the days of first instant messaging application, ICQ. Almost every computer connected to the internet has some kind of instant messaging software installed on it (most often MSN Messenger as it comes pre-installed with the windows operating system.) The vast majority of the instant messaging market is now shared between yahoo instant messenger, AOL's messenger, and the MSN messenger.
The features of the different instant messenger software applications are fairly similar and for many users the choice of instant messenger software has to do more with a service provider or operating system than anything else. For example, people who use AOL as an internet service provider are likely to use the AOL instant messenger, people who use yahoo for their email account are likely to use yahoo instant messenger etc. . .
Recently, all of the instant messenger downloads have included advanced features like the ability for voice and video communication. The main thing a person should keep in mind when looking at instant messenger downloads is the type of network protocol the instant messenger software uses. Unlike different cell phone networks, for example, where people on different network can communicate with each other seamlessly, instant messaging software uses proprietary protocols that don't allow users to communicate between networks. In other words, if you use yahoo instant messenger, you can only chat with other users who use yahoo instant messenger.
These different instant messaging protocols are the reason instant messaging does not yet function like the telephone - i.e. with everyone connected to the internet being able to easily and instantly communicate with each other. This is likely because there is a lot of money to be made in the business of communication, and large communication companies don't have it in their interests just yet to allow instant free communication across the globe. The transition, however, towards a globalized network is beginning to happen, albeit slowly. Recently it was announced that yahoo instant messenger software will share its protocol with MSN Messenger (which uses the .NET framework) and vice versa. With yahoo instant messenger software able to now communicate with the MSN network, some strong competition will be created for AOL, whose AOL instant messenger (AIM) is currently has the most users.
It's not clear whether this integration between yahoo instant messenger and MSN will initiate further towards a unified instant messaging protocol, or simply result in the number of different instant messaging protocols being reduced. With the introduction of voice communication of yahoo instant messenger and the like, however, it is certain that online instant messaging will keep making inroads into most people's everyday communication, and will likely become a strong competitor to the traditional dominance of phone communication.