exactly ctrl+c / ctrl+v
obviously ctrl (and alt) are the one you choose (by default, Calendar and Addresses).
Keep in mind that copy&paste implementation strictly depends on the software in which you are trying to use the feature. So, if you have, for example, a desktop manager and a file explorer uniform in protocols (or better, managed by the same software, like ROX) you can expect an easy copy&paste between the two. But if ROX implements a custom copy&paste protocol, you can bet on the fact you won't be able to copy&paste between it and other software. Anyway, what you can do is try to discover effectiveness in different cases.
You must consider, though, that in GNULinux and similar OS is quite normal having the possibility to copy and paste only if the app you copied from is left open. If you copy some test in Firefox, and close it before pasting, the copied text will be lost
bye