Screen Sharing is a built-in VNC client for Mac OS X. It is integrated with Finder, but can be manually launched from System -> Library -> Core Services.
With Screen Sharing in OS X Lion, you can remotely log into a Mac with any user account on that computer and control it, without interrupting someone else who might be using the computer under a different login. PCs are supported, too.
Screen Sharing in Lion supports full-screen mode, so you can observe and control a remote Mac using the entire screen. It feels as if you are sitting right in front of the other Mac. More Info »
With Lion, you can now use an Apple ID to authenticate with another Mac running OS X Lion to start a screen-sharing session. This is perfect for giving others access to your Mac without creating separate user accounts. Simply add their Apple IDs to the list of authorized users, and they can log in with their credentials.
The new Observe-Only mode lets you watch a remote computer without controlling the mouse or trackpad movements.
In Lion, Screen Sharing includes a new toolbar that provides access to useful tools and settings such as screen resolution, Clipboard access, and screen capture.