Hello Steve -
For Wide Area Network connections, we have 2 major categories:
- Dedicated
- Switched
For "dedicated" a company pays for a leased line between site A and site B, and the company can use it however they want. This is an expensive option.
For "switched", we have 3 options:
- Circuit switched (such as an old, old switchboard telephone system).
- Packet switched (such as Frame Relay)
- Cell Switched (such as ATM)
For circuit switching, imagine an old, old telephone system, with a switchboard operator. (The operator doesn't have to be old 
)
)
When a call need to be place, they would plug in the correct cables, and you would have a "circuit switched" connection for the duration of your call, or until they unplugged you (tore it down).
For Packet switching, a service provider has possibly thousands of customers, all connecting to the frame relay cloud, and the service provider will forward the packets (packet/frame switching) within their cloud through a variety of paths. The end result to the client, is that the traffic flows from one of the customers sites, to another, using the logical PVC (Permanent Virtual Circuit). The customer is in the dark about the details in the cloud, and several technologies may be used in the cloud, (including ATM).
For Cell switching, it is the same scenario as frame relay, except the provider is offering ATM instead of Frame relay to the customer. We still have logical paths from the the customers point of view, but within the ATM cloud, the provider can move/switch the cells the over a variety of paths.
Hope that helps,
Keith
