Desiree,
Usually you can make a Segmentation based on just about anything. Typical uses are Regional breakdowns (Americas, Europe,...) or Buyer Personas. I've seen all sorts of options.
So you don't need to be vague, but you do need to have mutually exclusive Segments because the selection occurs as a waterfall. If you meet the first criteria, you are part of that Segment and not part of another.
So you could have a Segmentation of Member Level by Region:
-Gold+Americas
-Gold+Europe
-Silver+Americas
etc
You can also accomplish the same thing with a smart list, but smart lists are slower to run and don't support Dynamic Content.
Yes, you can create a single campaign to include one or more Segments (ideally you would do this!) and then your email would be setup to display content to each Segment as appropriate.
See
http://www.marketingrockstarguides.com/how-to-create-marketo-segmentations-1371/