If I have data regarding advertising traffic (number of impressions displayed), revenue from these impressions, browser, country, device, etc., what kind of statistical test can I apply? I want to be able to say, for example, that traffic from country a is more valuable than traffic from country b, or traffic from mobile devices brings more revenue than traffic from desktop devices.
First you must determine whether your data are categorical, ordinal or interval. Then you must consider how many independent variables you will look at at once.
Next, considering the aforementioned matters, use UCLA's Institute for Digital Research and Education chart:
I also recommend Reasoning With Statistics, by Williams, as it covers the basic approach to statistical analysis in a concise format.