I was hoping someone could explain the levels Ingress uses. For example, do L2 XMP bursters only take out L2 resonators, or can they take out L1 resonators as well?
Each level of XMP make the weapon more powerfull.
Each level of Resonator make it more resistant (and increase range of potental links).
Here you can find a table with the power of each level of XMP Burster (source) :
EDIT: Since 18 January 2013 there are some new range values :
Or a nice image (Outdated for the moment) :
Here is a summary of the different levels :
To supplement @Cedric's answer, the decay rate is not known at present but is clearly not linear. Anecdotal evidence suggests a major difference between standing within a meter of a resonator vs 5m away and only a minor difference between 10m and 15m away. There's a reddit thread with discussions and a much more plausible graph of damage from WolframAlpha. It's not clear how they obtained this information or how precise it is but it corresponds much more closely with my experience playing the game than the linear assumption.
This shows two hypothetical curves for an L8 burster only, with the old range. The numbers may not be precise but the steep power law/exponential shape seems plausible for all levels.
From a physical point of view, conservation of energy would indicate a dropoff ~ r^(-3) because it is a 3 dimensional phenomenon (even if its effects are only registered in two dimensions). A frictional term would add something like e^(-r/r_o) which would generate something more like the attached graph. Though the game's logic is in flux, I think they will likely keep with the pseudo-physical interpretation because it will tend to "feel more natural" than a linear response.