Made by Native Instruments, the FM7 is a VST (and DXi, ASIO and various others) software plug-in version of the Yamaha DX7. It can emulate it as close to perfectly as most people feel a need for, and surpass it in several ways, such as offering many waveforms as a starting point rather than just sine waves.
As far as plug-ins go it's fairly expensive, but a real DX7 second hand would set you back a lot more money, and plug-ins seem to be a lot less hassle to use than "real" synthesizers these days.
As is the case with any synthesizer, hearing it is infinitely more useful than reading about it. Luckily there is a free demo you can download from the official site that lets you try it out for a while. It's worth looking into if you write music on computers and want a DX7 but can't afford or find one.