If you haven't ever played Tower Defense, then you've probably been doing actual work at work rather than frittering away your office hours playing web games. Curse you and your "work ethic."
Fortunately, Fieldrunners offers the perfect introduction to the distraction formerly known as Tower Defense. The object is to protect your base from an onslaught of enemies by placing weapons along a central pathway. As enemies waltz by, they get chewed up by your defenses. Touchscreen support makes it that much easier to plunk down defenses, of course.