I have had this app since the day I got my droid (it was the first app I actually downloaded), so here is a brief breakdown of the Tricorder app and what it does.
First thing you need to know is that it is skinned to match TNG, with audio to match. If you are looking for a Bones or Spock moment, sorry. Now, the settings, in order they appear on the screen:
Grav: This is the acceleration monitor. If you move your droid around, it will register the amount of force you are using and the direction you are going. I haven't tested this in a car yet, but it might be interesting.
Mag: This is a 'magnetic flux' reading. Whether it is standard magnetic fluctuations or emf it is detecting, you will get readings near anything electronic. It shows both vector and magnitude, so you can see spikes as they occur. Right now, with it sitting on my laptop, it is reading an 80-81 magnitude. The exact same spot with my emf gauge bounces between 1.7 and 2.2 gauss.
Aco: This is the acoustic reading. Basically, what it does is register your background noise at any given moment. It shows waveform, spectrum and power (in dB).
Geo: Geographic; this is basically a compass/gps system. It shows both your compass (which is easily confused if you are indoors), gps, and 'cellular' settings, though cellular and gps are pretty much the same.
Ems: This shows your cell connection (carrier and what I assume to be conversations or carrier line transmissions like market downloads, txts, and the like) as well as all wifi signals in the area. It indicates wifi strength and which you are connected to, and which require passwords.
Sol: This is the solar activity monitor mentioned earlier. It shows proton and electron levels for roughly the last 2 months, though it is typically a few days behind (it says Dec 21, but it is Dec 28 as I type this). Tapping the sun flips you through what appear to be a number of filters, ranging from uv 304 thru uv 171, visible light, magnetogram, and a couple corona settings. It also shows solar flares, sun spots and rf fluctuations.
I hope this helps.