Knowing who your competitors are and what kind of time and monetary investment they are making into their online marketing is nearly as essential as implementing your own marketing strategy. If you have a good idea of what you're up against can make better decisions about where and how to invest your own time and money. You will also have a decent indicator of how quickly you might be able to see results from your efforts...
When analyzing your competition there are three types of competitors you should review: Those naturally ranked in the search results, those dominating the paid ad results, and those that are offline but targeting your same audience.
Naturally ranked competitors
Paid Placement Competition
Offline Competitors
Comment: see original article for author's comments on each of the noted three factors.