Southwest Airlines will start charging $35 to check a bag for many passengers on reservations made after May 28.

Southwest Airlines has put a price tag on checking bags, a coveted free perk that is about to disappear as part of substantial changes the carrier is making to its service.

Times have changed, and one of the pillars of its business model -- "bags fly free" -- has finally fallen by the wayside. Here's what travelers need to know.