Smart Gate is a general purpose noise gate. To put it simply, a "gate" in audio terms means shutting off sound when certain conditions are met. In the case of the Smart Gate, when the input volume from your guitar goes below the selected level (set by the threshold knob), the gate will close. However, you have the choice of closing it completely (full) or only at some frequencies which are most commonly found in noisy guitars/pedals (hiss). The gate also intelligently closes at different speeds depending on whether you are sustaining a note (the gate will slowly close until your sustain runs out) or whether you suddenly stop playing (gate will immediately close). The obvious drawback is when you are actually playing with your high gain / noisy stuff, the gate can't do much. It only takes effect when you are playing softer/slower or stopping.
Noise Clamp is also a simple noise gate, but it specifically targets noise caused by your pedals (instead of your guitar pickups/wiring etc). How it works is you connect guitar to INPUT, SEND to your noisy pedals, RETURN back to the Noise Clamp, then OUTPUT to other pedals or your amp. Let's say you put a distortion pedal in the send/receive loop. When the distortion pedal is off, the Noise Clamp doesn't do anything. When the distortion pedal is on, the Noise Clamp compares your clean signal (coming from the input jack) to the distorted signal (coming back from the return jack), and filters out what it thinks is unnecessary noise in the distorted signal before going to the output.
Note that these are based on general assumptions about how noise gates work and I don't have extensive experience with either pedal.
As to where to put a noise gate, it depends on the source of the noise. If your guitar pickup or wiring is noisy (e.g. most single coils, unshielded wiring), you might want to put it first after the guitar. If a specific pedal is noisy, you could put the noise gate right after that pedal. For the Noise Clamp you'd put it at the same spot "parallel" to the noisy pedal.
Before you think about noise gates though, you should troubleshoot your current setup if you think it is noisy. Sometimes it's just one patch cable or your power outlet that is causing the noise, and it's better to solve the problem at the source rather than add a noise gate which can only help partially.
