10.4.11
This number represents the kernel revision, and is changed very rarely, and only when major kernel changes are made, or an entirely new kernel is used.
This number means there are major changes above the kernel, usually to the APIs or user interface. This number changes more often, usually once every year or so, and is considered a "major" release in that every time this number changes, you must pay for the "new" OS.
This number can change quite frequently, and will always be a free update. It changes for various reasons, but usually for security fixes, bug patching or other minor changes that don't change the kernel or user interface.
Thus, you currently have 10.4.11, the latest and greatest revision to Tiger. If you want Leopard, you need to purchase it, and then you will have 10.5.2. Note that eventually 10.6 will be released, but not for at least another year (or two.)