This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
When I was new to Rails I was pretty confused by the term "migration" because I always understood it to mean "Labor intensive SQL project to move DATA from location A to location B."
In Rails, "migration" just means "tweak the database schema a little bit" and sometimes it also means "move some data around while changing the schema" but the moving part tends to be fairly rare - pretty much only when doing a refactoring to the domain model.
Since Rails is an agile, build-as-you-go framework, migrations typically just add more tables and columns to an existing schema.