What's the difference between the JSDK and the JSWDK? And what's the current version?
The kit for developing servlets, containing the Servlet API classes and tools, used to be called the Java Servlet Development Kit (JSDK). Then Sun renamed the Java Development Kit (JDK) to the Java 2 Software Development Kit (J2SDK). Since J2SDK sounds a lot like JSDK, the Servlet team renamed JSDK to JavaServer Web Development Kit (JSWDK). (They also added support for JSPs.)
Here's where it gets confusing. When they renamed it, they also renumbered it. So the JSWDK 1.0 is actually more recent than the JSDK 2.1. It's also confusing that when people want to develop servlets, they have to download something called a JavaServer Web Development Kit, which sounds like it should be used to develop servers, not servlets.
In short: Make sure you look for the "W"!
Tomcat contains code based on JSWDK, as well as a lot of new stuff, and it will be the supported Servlet and JSP implementation from now on. No further work will be done on JSDK or JSWDK.
See What version of the Servlets or JSP specification is supported by my favorite servlet product? for a summary of the spec versions supported by each product.