The threshold run is what is most commonly thought of when you talk about tempo runs. The name refers to the idea that you are running very close to or right at your anaerobic threshold. That is the point at which your body begins to produce more lactic acid than it can use as fuel and the PH of the blood begins to rise sharply. It is a bit of a false hood in that the exact pace will change as you tire but essentially the idea is to run at this pace.
There are a number of ways to determine this pace for yourself ranging from blood tests during activity and VO2max testing to calculators and charts based on workouts and race performances. The best guideline I have found is that it is roughly the pace you would expect to RACE for a 1 hour race. For me I use half marathon pace, All my halves have been in the 1:04 to 1:08 range. For someone else this might be 10mile race pace or 10k. The thing to remember is the pace varies a good bit with level of exhaustion as well as changes in weather or terrain so always trust feel over the watch.
These runs should be run steady or with a slightly negative split, but if your last mile is more than 5 or 10 second faster than your first mile than you are doing a progression run and that is fine but it is a slightly different stimulus. The threshold tempo is about the steady pressure on the cardiovascular system. These runs can vary in distance greatly. It largely depends on what kind of an effort you can get out of yourself in a non-racing situation. I myself can almost never go more than half an hour like this. In contrast a few years back Ritz ran a 10 mile tempo run at his half marathon pace a few weeks out from running 3rd at the World half marathon championships in 1:00:00. So he was doing 45mins. That is about the max you see and most people cannot produce an effort like that in a workout without a taper, race day adrenalin etc..
The most important thing with these workouts is that you are better off going too easy than too hard. If you push too hard and go into anaerobic running your body does not receive the message that the aerobic threshold failed. The idea is to lock in at a steady effort just below the threshold and exhaust that system. As to 'embarrass' it so your body will make growths there to address the problem. If you are a bit too slow this will mean you run a bit longer in the workout to accomplish this but it will be accomplished. If you are too fast than you simply won't see much improvement.
Like all tempo runs I suggest you improve the workout by increase the volume before the pace. I would suggest starting with about a 20minute effort and building up by 5 to 10 minutes each time until you are running 30 to 40 minutes. Than return to the original distance and increase the pace by 5 seconds per mile or so. You should find the effort is the same.
These tempos are most specific to 10 mile to half marathon racing for most athletes but the anaerobic threshold is the single most important marker of fitness for all distances from 3k to the marathon. In all those events your race specific fitness can be thought of as being built off of the anaerobic threshold. These workouts should be done throughout the training cycle and they should be a main focus of the pre- specific phase, or early season period of your training.
This workout builds the ability to run on the edge of disaster, pain, acid, oxygen debt while maintaining control. Effective distance running boils down to the ability to run fast without falling into those things so it doesn't take much imagination to see how key a session like this can be to your reaching your full potential.