The fundamental force governing TdF stage racing is wind resistance (drag). Riding solo, the difference between the best and the worst TDF rider is around 10%. Riding directly behind another cyclist, or diagonally behind in the case of cross winds, saves 20-30% of your effort at typical flat-road racing speeds. The best cyclist will easily lose out to the worst cyclist if the worst is allowed to follow/draft and conserve energy. The key strategy to understand about drafting is that the percentage of energy saved greatly decreases as speeds decreases. Wind resistance is roughly proportional to velocity squared. Going up a steep hill, fighting a strong headwind, or riding on cobblestones slows you down and reduces the effectiveness of drafting. That's why on flat areas most riders stick to a big pack, while going up mountains, riders spread out.
There are many competitions, not only the GC. The GC is the general classification, the total time for riding the 2000+ miles of the race. It's the biggest prize of the TdF but there are many others:
- Green jersey, for the overall best sprinter
- Polka dot jersey, for the overall best climber
- White jersey, for the best GC result for a rider under 26 yr
- Winner of each day's stage
- GC (yellow jersey) leader after each stage
Winning any of these is a big deal and is a career highlight for the winner. For the lesser teams, winning just one stage would make for a successful Tour.
The TdF is a team sport. The majority of the 198 riders on the 22 teams of this year's TdF have no chance of winning any of those prizes--not because they're not good cyclists, but because they are support riders called "domestiques", whose purpose is to support the top riders on their team. They'll do whatever is needed to help the top riders conserve energy and change the pace of the ride as strategy dictates. If the support riders are out of range to help their teams' top riders, then they need to conserve their own energy to ride another day. In individual time trial races they'll ride at 90% of their max effort since all they need to do is go fast enough to avoid disqualification.
Energy conservation is critical when you're racing 100+ miles a day. Riders bulk up before the race, eat around 7000 calories a day for the three weeks of the race, but still end up looking deathly gaunt towards the end. Energy you spend unnecessarily today will detract from the rest of your race. This is why you won't see any GC contenders going for sprint wins, and you won't see sprinters trying to finish in the front group of a mountain stage.
There are official rules, but many rules are unwritten. The race has evolved into one with a sort of road honor, where racers don't take direct advantage of the mishaps of others. A rider will be severely scolded if they attempt a breakaway right after a big peloton crash. If a rider violates these rules of honor too often, they'll be out of a job. Everyone is competing, but everyone also depends on else. GC contenders want to win because they're the strongest rider, not because their opponents crashed.
Flat stages are a battle between sprinter-led teams and small teams. Both teams mainly go after individual stage victories. Sprinter teams win by having races end where everyone's in a big bunch; they'll try to get to the front towards the end of races, and ride at maximum speed leading one rider at a time until they launch their best sprinter just before the finish line. Small teams tend to try breakaway victories, where small packs of riders attempt to leave the big pack early on and try to maintain their lead to the end. Though the breakaways are usually caught, they occasionally escape and win, but at the very least they provide some exposure for the team sponsor. Sprinter teams are the main force trying to bring the breakaway back. GC-focused teams typically try to stay out of trouble and conserve energy in the flat stages.
Mountain stages and time trials are where the GC is won and lost. Since low speeds decrease the effectiveness of drafting, much more separation gets created on the mountain stages, especially when the stage ends on a climb. The only winner in recent history who wasn't one of the top time trialers was Marco Pantani in 1998. This year's race only has one time trial, giving a lesser time trialer like Andy Schleck a better chance of winning the race.
Strategy is a huge deal. Most teams have multiple goals and the methods to achieve them are often conflicting. On occasion teams have had multiple riders at the top of the GC, most recently in 2009 when teammates Contador and Armstrong finished first and third. This inevitably results in conflicts because team resources need to be distributed between the two. One may even be ordered by the team manager to slow down to assist the other. The really hard part is figuring out what you think the other teams will be doing and how to react to that. Game theory folks could go nuts studying the Tour.
If all else fails, enjoy the amazing camera work and European scenery.