OS X Lion introduces "Natural Scroll", which is another way of saying two-finger scroll will work in the opposite direction that it did before. This is meant to make the experience more tablet-like, in that it's almost like you're pulling the screen in the direction you want it to scroll, rather than pushing towards the direction you want. Just look at the videos in the system preferences for Lion if it's confusing.
The problem, for many of us, is that it changes the direction of the mouse wheel as well. I like the natural scroll idea for the touchpad, but I want the mouse wheel to continue to scroll the page up when wheeled up, down when wheeled down. Disabling natural scroll for the mouse will disable it for the touchpad, and vice-versa. Bit of an oversight in my opinion.
The solution is the utility Scroll Reverser, developed to give pre-Lion users natural scroll. Post-Lion users can use it to selectively enable (or disable) natural scroll for the touchpad or the mouse, without changing both at the same time. Apple should have thought it through, but luckily we have good programmers to bring us workarounds.















