Valve issued an update for Team Fortress 2 earlier this week, full of bug fixes and gameplay tweaks. One of the more substantial, was a change to TF2's "critical hits" system, which Valve explains.
For those unfamiliar with how Team Fortress 2 handles "crits," all you really need to know is that the game hands out increased damage on attacks on occasion, which results in glee for the damage given, fury for the damage receiver. It's not exactly random, as Valve's Greg Cherlin explains in great detail on the official TF2 blog.
Basically, the new gameplay changes make more sense of the critical hits system, reducing the frequency of increased damage as well as making crits more "skill / performance based."
That's pretty good news for long-time Team Fortress 2 players, folks who may feel regularly cheated, then summarily enraged, by the odd crit rocket. It's also good news for TF2 noobs, who might be wondering what that thudding sound is when they're being filled with bullets and don't understand why their grenades have that extra glow.
For anyone who wants to understand the system a little better, particularly under the new rules, the algebra that makes it the crit magic work is pretty well explained by Valve. It might not change your game, but it's informative at the least.