Lion, the latest version of Apple’s OS X has taken a big step to improve security. Sandboxing and the principle of least privilege are being used to contain the effect of future security holes. Sandboxing creates a restrictive shell around a program and limits what actions it can take on your system, from writing files to accessing the camera. The principle of least privilege is being followed to split programs into smaller, more restricted chunks. For example, the part of the Safari web browser that displays web pages is being split off from the rest of the program since more security holes are typically found there. That chunk of the program has an even more restrictive sandbox around it, further limiting the risk to your system if a security hole is found. I definitely recommend the $30 upgrade to Lion, just make sure all your programs are compatible before installing it.
http://www.theregister.co.uk/2011/07/21/mac_os_x_lion_security/