Apple has released an update to its Java runtime for OS X, which is available for OS X Lion and Snow Leopard operating systems. The updates address some "reliability, security, and compatibility" issues involving the current Java implementations in these operating systems, if people have chosen to install them.
The largest fixes in this update address problems where code in Java applets could be run outside of the Java sandbox and thereby compromise the security of the system. Sandboxing is a method of isolating running code that has received a bit of attention recently.
Starting with OS X Lion, Apple has stopped supplying a Java runtime with the operating system, but Java can still be downloaded automatically if you run an application that requires it. Addtionally, Apple has announced it will not be maintaining an in-house version of Java for OS X, and instead as with other operating systems Java will need to be downloaded and installed from Oracle.
The latest Java updates can be downloaded via Software Update if you have Java installed, but if not then you can get them from the following locations:
Do keep in mind that Java is not required to run OS X, and if you do install an application that requires Java then the OS will automatically download it from Apple.