A crashed sprinter inside the final three kilometres will not win the sprint, but avoids being penalised in the overall classification.
The former tend to be raced on relatively flat terrain, which makes it difficult for small groups or individual cyclists to break away from the peloton—there are no big hills to slow it down.
In the final few hundred metres, a succession of riders "lead out" their sprinter, riding very hard while he stays in their slipstream.
Sprint stages rarely result in big time differences between riders (see above), but contenders for the General Classification tend to stay near the front of the peloton to avoid crashes.
(If the stage ends at the bottom of a mountain that has just been climbed, riders have the chance to descend aggressively and catch up to anyone who may have beaten them to the summit.)
For this reason, the mountain stages are considered the deciding factor in most Tours, and are often attended by hundreds of thousands of spectators.
Generally, these riders form a group known as the "bus" or "autobus" and ride at a steady pace to the finish.
Their only goal is to cross the line within a certain limit—usually the stage winner's time plus 15% – or else they'll be disqualified from the race (at the discretion of the officials; on rare occasions a lead breakaway becomes so large that the entire peloton falls that far back and would normally be allowed to remain in the competition to avoid having only a small field still in competition).
The Giro d'Italia has had a reputation of labeling selective, very difficult stages as merely medium mountain.
Lastly, a handful of stages each year are known as being "good for a breakaway"—when one or a few riders attacks the peloton and beats it to the finish line.
Breakaway stages are where the rouleurs, the hard-working, all-around riders who make up the majority of most teams, get their chance to grab a moment in the spotlight.