Tiffany,
If someone "likes" a message is the person who wrote the message
notified? Or would they only know by going to the message online and
checking there?
Unfortunately the latter, Likes are all but invisible at the moment.
There is a long-awaited "notifications overhaul" which should provide notification by web and optionally by email to the post's author, and maybe to others. As well as notices for a host of other events.
Also, what is the difference between "rich" and "plain" formatting on
the digest options for email delivery?
Rich is HTML formatted, with active links in the TOC and under each message.
Plain is plain-text, the TOC does not link to the messages further down, and the links to actions on site are spelled out as URLs which may (depending on your email interface) be linkified or you may have to cut and paste them to your browser.
Shal
Shal Farley