Ask any five IT specialists what cloud computing is, and you're likely to get five different answers. That's partly because cloud computing is merely the latest, broadest development in a trend that's been growing for years.

Cloud computing is the most recent successor to grid computing, utility computing, virtualization and clustering. Cloud computing overlaps those concepts but has its own meaning: the ability to connect to software and data on the Internet (the cloud) instead of on your hard drive or local network.

To do anything with a PC 10 years ago, you needed to buy and install software. Now, cloud computing allows users to access programs and resources across the Internet as if they were on their own machines.

In the Beginning

First, there were mainframe computers, then minicomputers, PCs and servers. As computers became physically smaller and resources more distributed, problems sometimes arose when users needed more computing power.

IT pros tried clustering computers, allowing them to talk with one another and balance computing loads. Users didn't care which CPU ran their program, and cluster software managed everything. But clustering proved to be difficult and expensive.

In the early 1990s, the grid concept emerged: Users could connect to a network, much as they plugged into the electrical power grid, and use service on a metered-utility basis. Thus, people began speaking of utility computing .

One problem was where data was stored. Grid nodes could be located anywhere in the world, but there could be significant processing delays while data stored at other locations was transmitted.

Also, grid or cloud computing means users and businesses must migrate their applications and data to a third party or different platform. For enterprises with huge investments in existing software and operational procedures, this has been a real barrier to adoption of these shared technologies. Other significant concerns include data security and confidentiality.

Why It Works

Critical to the success of cloud computing has been the growth of virtualization, allowing one computer to act as if it were another -- or many others. Server virtualization lets clouds support more applications than traditional computing grids, hosting various kinds of middleware on virtual machines throughout the cloud.