The Sun JDK consists of tools for building and testing Java applications (javac, java, appletviewer, etc.), and includes a copy of a redistributable runtime environment (which also contains the java executable).
See also Sun Java JRE for information about the runtime environment specifically.
Note that (a) the Linux version of the Sun JDK runs just fine under Linux and (b) there are other Java compilers, JDKs and Java virtual machines for Linux. The Linux 1.5.0 JRE is a 15MB download, only slightly bigger than the Wine .deb package. The main reason we track Java support is because it's an interesting test-case (and because the source-code is available).