Hi Robert.
Basically, the X series is used as the Extreme Platform. In this case, the processor supports more pci-e lines, cache, cores and threads. You can use quad channel DDR4 memory and the socket is known as 2011-3 (for haswell-e). Only Core i7 Extreme Edition and Xeons processors can be used in this chipset. Finally, the worst feature is its price, the X Platform (processor + motherboard) is more expensive than others.
Z97 is the current chipset used for the regular consumer. This one come with less pci-lines and support only Haswell and Devils Canyon processors (Maybe Broadwell the next year). The range of processors includes the Pentium, Core i3, Core i5, Core i7 and some Xeon. The memory only supports DDR3 dual channel. This is a more cheaper than the X series in general.
In the case of the Z99 chipset that you asked, i never heard about it. Maybe you're referring to Skylake chipset, which is supposed to arrive in 2015 to support the new processor and DDR4 on the regular consumer oriented platform. This chipset has no official name till now.