If it is Google then they will tell you exactly why you were targeted...there should be 2 icons in the top right corner of the banner add, one is an X and the other is a triangle. Click on the triangle and the ad will change to have to buttons saying "stop seeing this ad" and "why this ad". Clicking on the "why this ad" button will show you the reasons. For example I saw an ad just now for a high end apartment and I clicked on it. The page explains:

This ad is based on:

  Your visit to the advertiser's website and/or app
  Google's estimation of your Household Income
  Your age group