Knobs are probably small volcanoes and erosional remnants.
Plains are probably degraded lava flows or sediments. Probably consists of aeolian deposits. The aera of the Mars Map within the Shockwave window can be printed. Also, a printable version of the entire map is available in the Printable Lesson Plan section's Printable Global Mars Map page. Viking Orbiter images of Mars can be found at the Center For Mars Exploration's Atlas of Mars site.