When people say "suicide hub" they mean a hub that has a cog screwed onto a freewheel with no lockring. This is obviously dangerous, because if you apply some backpressure, you can spin the cog right off the freewheel and you will no longer have a way to slow down your bike. Of course, if you ride with brakes, this isn't as much of a concern.
I thought that suicide hubs also included freewheel hubs that have a cog scewed on and a lockring that's held on with locktite. But maybe I'm wrong. Plenty of people use the locking and locktite method, but it's not such a great idea. If you do, definitely use a break or two.
I've never actually heard it referred to that until just a few days ago, here, but it's an old road hub with thread-on freewheel threads. You thread on the track cog...optional components are a bottom bracket lock ring and/or locktite (pick your color). The problem is all the threads are in the same direction. Pedaling forward tightens and backpressure...well...
A proper track hub has a set of clockwise threads for the cog and a smaller diameter set of threads running counter-clockwise to thread a track lockring on. Forward pedaling tightens the cog while backpressure "unthreads" (in reality, there should be no play) it against the lock ring which then tightens down in its own threads.
And even if you have a brake it can still be a bad scene. I threw a chain (which is essentially what happens if you unthread your cog) and ended up facedown in the middle of a major intersection when the chain jammed the wheel.