Easy question...

Tonight I noticed a new (or what I think is new) place in the left finder Window.(I'm sure it has a more technical term). It was a shared...device..or folder...had a long name...letters and numbers - and when I clicked for more info - it said Mac server. About 5 minutes later it has disappeared from the Finder window. What was that all about?

You were most likely picking up someone else's network temporarily. Do you live in an apartment building or in close proximity to other homes?

Make sure you turn off sharing on your side and if you're using a wireless net connection, use the best encryption that your system is capable of. (Probably WPA2)

Relative close proximity I guess...we have 1/3 acre lots here. I do use the max encryption - and I did note that the drive or device was shown as not shared. Interesting that it would show a device on their network...that I'm not connected to

What happened is that one of your neighbors got careless. That's why it pays to be careful about security.