The New York Yankees and Boston Red Sox are obviously big-market teams. The Kansas City Royals and Pittsburgh Pirates are notoriously small-market clubs.
The new collective bargaining agreement makes these distinctions more important than ever, because it best serves big- and medium-market clubs. It could conceivably create an underclass of teams unable to compete for reasons financial and infrastructural.
Most fans, though, might have a mistaken vision of what that means for a dozen or so teams around the league. That's roughly the number of squads whose designation (are they big-market, mid-market or small-market?) by most fans and analysts misrepresents the truth of the matter.
Why is it important?
Well, for one thing, half of the 30 current teams in MLB get some share of revenue from the other half, under rules established to encourage competitive balance at the very beginning of the 21st century. The 15 who get the money, though, are not the same 15 who play in the smallest markets, or who have the smallest revenue potential.
For another thing, like it or not (I do), baseball is still a relatively free-market enterprise. Teams can spend as they see fit, so things like market size and fanbase matter insofar as they can impact a team's ability to make the right decisions without regard to cost.
Here are all 30 MLB teams, ranked according to their market size, i.e., their overall revenue streams.