Two different processes at once?

I need some rapid help. Leaving on a trip tomorrow and need info in the MBA. I seem to have 2 computers in my iMac. I was going back and forth from the airline app to the email app to the notes app and minimizing things as I got quite a few windows open. Then I found the the screen jumping between 2 different sets of minimized items. The entire screen jumped, my desktop icons and background picture remained the same. When I closed everything on one screen I could not get too the other one that had 2 items open/minimized. What's happening? I recall several years back (and at least 2 OSs back) some how getting a second screen as if it was a second computer. Any relationship to today's problem?

Thanks.

Geoff