Off-site.
A basic principle of cloud computing is that you're accessing IT resources that are in a data center that's not your own. That means you don't buy the servers and storage, someone else does.

Virtual.
IT resources in the cloud can be assembled with drag-and-drop ease. Employing virtualization, cloud service providers let you assemble software stacks of databases, Web servers, operating systems, storage, and networking, then manage them as virtual servers.

On demand.
In the cloud, you can add and subtract resources, including number and type of processors, amount of memory, network bandwidth, gigabytes of storage, and 32-bit or 64-bit architectures. You can dial up when you need more, and dial down when you need less.

Subscription style.
These tend to be month-to-month deals, often payable by credit card, rather than annual contracts. Amazon charges in intervals of 10 cents per hour for EC2.

Shared.
For economies of scale (that's what cloud computing is all about), use a multitenant architecture to squeeze workloads from multiple customers onto the same physical machines. It's just one of the things that distinguish cloud computing from outsourcing and from hosted data centers.

Simple.
Many of the cloud service providers let you sign up and configure resources in a few minutes.

Web based.
Others might make this characteristic #1, but i put it last to make the point that there's more to cloud computing than the Web. That said, it does involve browser access to hosting data and resources.