Normally when you get a beach ball, something else is running in the background and needs to finish before it will give up the CPU.
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A persistent, recurring beach ball (officially called the "spinning wait cursor") normally does not happen solely due to background processing. E.g, you can start an extremely CPU or I/O-intensive process, then while it's running switch to Safari and Safari will not become a beach ball. OS X uses preemptive multitasking so the background process need not finish -- the dispatcher will give each runnable process a time slice.
The beach ball is caused when the foreground process does not handle its events within a few seconds. Several factors can cause that including programming errors in the app or an overloaded system or hardware problems. Of those a programming deficiency in the app is the most common. This is generalizing across all apps, many of which could use improved event processing. The programmer should never allow his app to have a spinning wait cursor under normal conditions. Rather a separate thread should be used to handle those events and indicate to the user via a progress bar or other UI construct that the app is busy doing something. More details:
https://en.wikipedia.org/wiki/Spinning_pinwheel
However in this particular case lori5060 said it happened even with Preview and Word. Those are generally well-written so it more likely indicates a system-wide problem, e.g, intermittently failing hard drive, memory leak consuming page file space, out of disk space, etc. She already checked disk space.
The other advice is good about using Activity Monitor to see what's running, reboot the system, running Disk Utility to verify the disk volume, etc. A persistent, recurring beach ball is not normal and usually indicates something else than the system is a little busy.