Hi Melinda, delegation is a loan of SP; in your Steemit Wallet this is reflected in the figure in brackets, so delegations to you are positive, those from you are negative. One difference compared to a loan is that the delegator always has full control so can undelegate at any time.
I don't think there are any rules, except to obviously keep enough SP to be able to interact with the blockchain, although if someone wishes to be away from the platform for some time, then they may delegate most of their SP so that it is earning in their absence.
Thank you. I assumed that was how it worked, but I wasn't certain. I'll keep working at building SP so that the day will come when I can be more generous!