I messed up -- and followed some internet advice a few days ago without totally understanding the ramifications. Specifically, in trying to stop the Finder from asking me to verify every time I wanted it to move or delete a file, I went to my home directory, looked at its permissions in Info (Read/Write for me, Read Only for Staff and Everyone), thought that looked like what I wanted everywhere, so I then selected "Apply to Enclosed Items".
I've recently realized that I probably shouldn't have done this, as various things have begun failing at an accelerating clip. Yesterday, iTunes refused to back up my iPhone and iPads; and ClipMenu started reporting that it can no longer save my clipboard history (not a huge problem, but a good example). Today many of my preferences no longer work, and I can't set/save them (e.g., Mail "forgot" my password and can't seem to remember it when I set it, Finder preferences don't stick, etc.).
So ... is there a way to reset the various file and group permissions back to their original settings? I've tried Disk Utility -> Verify (and Repair) Disk Permissions, followed by restarts, to no avail. Note that I'm obviously not a guru and not nearly as smart as I'd hoped, so any help you can give probably needs to be "dumbed down" a little.
P.S. BTW, in regards to preference problems, one message I get is 'Group differs on “Library/Preferences/com.apple.alf.plist”; should be 80; group is 0.', which goes away after the repair, but comes back again after I've tried (and failed) to change a preference.
--- EDIT ---
The "problem"may not be permission-related at all, but a corrupted keychain!
I used the Keychain Access utility to verify and then repair (eventually, after several attempts) the keychain, and all of the symptoms seem to have disappeared, for now. I don't pretend to understand any of this, but I'm glad it seems OK now.
Thanks to all for their patient and thoughtful replies.