Grass is the one tennis court surface that changes character significantly during the course of a tournament, especially the most famous grass-court tournament, Wimbledon, which goes for two weeks. Grass is, after all, a living thing, and there's only so much a tiny little plant can take of being stepped on by a world-class athlete running at full speed or digging in to stop running and change direction. On the first day of Wimbledon, the grass courts are a lovely green throughout. By the second week, huge areas behind the baselines and near the service lines are reduced to brown remnants of grass and a lot of bare dirt.
On a fresh, green grass court, the ball tends to bounce fairly consistently, but quite low and fast. As the ball hits the grass at the acute angle typical of most tennis shots, it bends blades of grass in front of itself, and laid down, they form a fairly smooth surface upon which the ball skids forward, encountering relatively few vertical protrusions to slow it down or push it upward. On such a fast surface, points tend to be relatively short; therefore, a grass-court generally provides the least exercise per match. Grass is tough on the arm, though, because the ball hits the racquet with more speed, and more speed generally means more shock and torsion.
The amount of torsion the arm suffers increases as the court gets more worn, because bounces become more unpredictable, leading to more off-center hits. Unpredictable bounces also introduce more luck into the game. A fast, unpredictable surface tends to discourage patience, as the potency of aggressive shots is enhanced and the ability to rely on steadiness is diminished, in part because topspin is a major tool for consistency, but it's less effective and more difficult to execute when the ball bounces low and more difficult to time when the ball bounces unpredictably.
Whether grass is fresh or worn, it tends to be slippery, and even slight wetness makes it quite unsafe. While hard courts can remain playable for several minutes and clay sometimes indefinitely in a light drizzle, play must be suspended almost immediately on grass.