Apple erred on the side of plenty in providing ways to start a screen-sharing session in Leopard. Leopard has four different built-in ways to start a session, all of which use a somewhat hidden program called Screen Sharing.
I also cover VNC, which isn't part of Leopard, but which Apple supports for backward compatibility—Leopard's screen sharing is built on top of VNC—and which can be used broadly among many Mac OS X, Windows, Unix, and Linux platforms.
The four ways of starting up a screen-sharing session rely on different mechanisms for finding or connecting to a remote computer and then proving your identity—authenticating yourself—if that's necessary:
iChat: Any two people with Leopard installed ...