The available plans differ by state, so I guess you are studying the plans in Michigan even if you are currently in Illinois. The U.S. Department of Health and Human Services
Medicare Prescription Drug Plan Finder even asks for your county.
Most prescription plans have a few (typically 3, 4, or 5) tiers, or price levels. Tier 1 might cover generic drugs and have the lowest co-pay. Tier 2 might be for "preferred" brand name drugs with a slightly higher co-pay, Tier 3 for non-preferred brand name drugs, and so on. The highest tier is often one for "specialty drugs", meaning the most expensive.
You might also want to read the forum thread named
What is the doughnut hole I have been reading about?