When a driver complains about congestion, she's talking about the cars that are backed up on the highway. When your sneezing friend complains about congestion, he's talking about his stuffed-up nose.
Use the noun congestion to talk about over-crowding; this comes up most often when people talk about traffic. If there's unexpected congestion on your route to work, you'll be late. You might also use the word to describe a crush of people: "The congestion in the subway was terrible — we were packed like sardines in there." The medical kind of congestion describes a build up of fluid, especially the kind you feel in your sinuses when you have a bad cold.