@tedd is right. Running is a more strenuous activity, but people who do lots of running over a period of years are more likely to have joint problems due to the impact of hitting the ground with each step. Biking removes this impact, but it works fewer muscles so you need to bike harder in order to get the same workout. I prefer biking, because I get too winded too quickly when I run, whereas I can last a lot longer (and go much farther!) when on a bike.
Swimming is great. Works pretty much every muscle in the body without hurting your joints!