Smear-Ripened Cheeses are softer cheeses that were made in lowland areas. These were cheeses that didn't need as long a storage life, as they were being made very close to where the markets where (cheeses made in highlands and the mountains often had to be able to last through the winter, until passageways down to the markets were safely passable again.)
A bacteria, often the bacteria called "Brevibacterium linens", is smeared onto the rind of the cheese.
While aging, the cheese's rind is washed to discourage mould growth and to give lots of moisture to encourage growth of the bacteria. The type of wash used is designed to create both a pH level which is beneficial for the type of bacteria that you want to encourage, while being hostile to bacteria that you want to discourage.
Port du Salut and Trappist cheeses are Smear-Ripened, as is Munster.
This is often confused with Washed-Rind Cheeses; with Washed-Rind Cheeses, however, the goal is to limit all bacterial growth and encourage mould.