OS X Lion will include built-in support for multi-touch gestures. While you don't need to use gestures to work with Lion, they can help you perform some tasks more quickly.
Multi-touch Supported Mac Notebooks
Notebook Macs have included support for multi-touch gestures since late 2008. Earlier models may lack complete support for multi-touch, or may not support gestures at all.
If you own an older model Mac notebook, or a desktop Mac, you can acquire multi-touch capabilities by purchasing an Apple Magic Trackpad. If you're used to the trackpad built into your notebook, the Magic Trackpad may be a pleasant surprise. You get the same multi-touch capabilities on a trackpad that's 80% larger than the one built into the MacBook Pro line of notebooks.
Lion will continue to support the Magic Mouse as a multi-touch device. However, as currently supplied by Apple, the Magic Mouse doesn't support three-finger gestures, which Lion can use for accessing specific functions and moving between various applications. Only Apple knows whether the current Magic Mouse can be updated to support three-finger gestures.
Using Gestures in OS X Lion
OS X Lion will start life with a handful of gestures, but I expect additional gestures will be added over time, both by Apple and by third-party utility makers who will create custom gesture systems.
The standard gestures for Lion include:
Three-finger vertical swipe: This is used to access Mission Control, a combination of Expose, Spaces, and windows and application management. You perform the three-finger vertical swipe by placing three fingers on the multi-touch device, and dragging them upwards.
Three-finger horizontal swipe: Used to switch between currently running applications. This gesture will be especially useful when working with full-screen apps. You perform the three-finger horizontal swipe by placing three fingers on the multi-touch device and dragging them to the left or right. The direction you move your fingers will determine whether you'll move to the previous (to the left) or the next (to the right) application.
Two-finger scroll: Scroll bars in Lion aren't always visible; they tend to disappear to provide more space for a window's content to display. To scroll within a window, as opposed to just moving the pointer, place two fingers on the multi-touch device and drag the fingers up (scroll up) or down (scroll down).
Tap to zoom: This is a two-finger gesture; tapping the multi-touch device with two fingers will zoom in at a predetermined percentage.
Pinch to zoom: Placing two fingers together and then moving them apart will zoom in. Likewise, placing two fingers apart and drawing them together will zoom out.
Swipe to navigate: This is a two-finger gesture, performed the same way as the three-finger horizontal swipe. The two-finger version is used to move forward and back through web pages, document pages, and any application that has a page style of navigation.