What is it and what does it mean for the normal pc user (is it useful)?
Cloud computing is also called utility computing because you pay as you go kind of service. It is useful because you can access the same file anywhere. Here is a youtube video that explains the basics of cloud computing. http://www.youtube.com/watch?v=flYZTVpFy7o
Webmail is techincally cloud computing. Cloud is nothing more than an expansion of an existing model with a shed load of marketing crap posted on the top. Hell, you've been able to rent a VM running your OS of choice for many years now and access it from any net connected device.
It's not much fun if you hve a restricted Broadband (or DSL) package from your ISP. In the UK where we have ADSL - A for asynchronous - our upload speed is around a tenth of our download so it takes ages to put data into the cloud. Then if your package has download restrictions, you put a load of data up there then you find you've over this months limit and can't get it back.
It's one of the things people seem to forget when they work from home. I've had it a few times in work where people complain that their connection from home is slow and blame the VPN connection as 'I can stream HD video fine' etc.
It is possible to sacrafice the download and get DSL (without the A) but only if you go for business lines in most cases.