shared cck fields

I'm not sure if this is simple question or not, but I'm a little bit confused.

I am planning on developing 4 drupal sites that are essentially pretty much the same in terms of what they do, they will look different but will more or less do exactly the same job as each other with the odd exception. What I'd like to know is if it is possible to feed the sites from the same database and most importantly can the sites share the same cck fields 'if and when' they are required, or do you need to either share all fields or non at all?