Well, I could try to explain this, but if you need it, you already know it. Here's the official description:
"The Microsoft SDK for Java, version 1.5 provides the tools, information, and samples that you need to develop great Java programs and applets. Because the SDK contains Microsoft's latest Virtual Machine for Java as well as the latest tools, packages, and other enhancements, it is the one place you will always turn to first for your Java programming needs.
Download the Microsoft SDK for Java now and see the Release Notes for a description of the contents of the SDK and instructions on how to begin making your own Java programs and applets. When you are ready to learn more, see the documents listed \[on the Web page\] for details about the features and services available to you in this SDK."
Microsoft includes a Netscape Navigator plug-in with the SDK that replaces the Java virtual machine that Netscape offers.
Microsoft has also released an interesting little article that was written by VP John Ludwig, discussing Microsoft's plans for Java. If you're interested in Microsoft's plans for the language and are worried that the company intends to "ruin" Java, it's a must read.
Want more information?
Microsoft SDK for Java
Start reading about the Internet: "Java: No Jive"