There are loads of correct answers and a few wrong ones in this thread so I'll clear them all up. I have the official PES5 guide which explains them properly:
Response: How fast the player reacts to loose balls + how fast the player reacts to the commands you input (although the difference must obviously be milliseconds - it would be no fun to have a 70 response player take an extra second to react to the ball )
Aggression: The player's willingness to attack and make forward runs. The other thing that makes a player more likely to make forward runs is the 'reaction' special ability star.
Technique: The player's ability to control a ball, and how well the player can control the ball when playing one touch passes. Ever noticed how when you try to play some slick one touch football with crap players, after about three passes the ball will be bobbling around and not going along the ground? This is because you're trying to play one-touch with players that have poor technique.
The 1-Touch-Pass special ability star has no effect on a player's ability to make first time passes. In the guide Seabass admits this is a mistranslation and it actually is meant to be first time direct shots e.g. volleys. When I read this I felt stupid that whilst editing my game I had been giving this star to holding midfielders like Makelele
Lastly, high technique also improves a player's chip shots. (I'm as surprised as you)
Attack: The guide says the attack rating isn't that important any more because there are so many other stats - in earlier games with less stats it was more of an umbrella attribute representing many characteristics. The attack rating is basically the ability of an attacker to lose his marker (off the ball I think).
Defence: A high rating makes a players tackles (block tackles and sliding tackles) faster and more accurate (i.e. more likely to take the ball and not the man). It also makes players get to their feet faster after making a sliding tackle.
The Sliding special ability star also enhances speed and precision of slide tackles.
Mentality: A player with high mentality does not have the physical side of his game affected so much by getting tired. The stamina stat represents how quickly a player becomes tired (how quickly their stamina bar drops) and once the in-game stamina bar drops under 50% the player's physical stats (i.e. speed, acceleration, jumping) suffer, although there are no stat drops until that 50% mark.
A high mentality serves to reduce the amount that a player's physical stats drop once a player becomes tired (has less than 50% on their stamina bar).
Balance: How strong a player is in physical challenges with opposing players, and the chance that they will stay on their feet in physical challenges (that is the chance that they are not knocked over [do not confuse being knocked over because of being too weak with being fouled] - not the chance that they will dive).
A few others that I don't think have been mentioned but aren't that clear:
Agility: A higher agility makes a player take fewer animation phases to perform an action, meaning the action is performed more quickly.
Penalties special star: Having this star will only affect a player's ability to successfully hit the ball into the top of the net - it makes no difference to the player's ability to hit the bottom corners.