There are really 3 baskets in which competitors fall into.
They're the direct competitors, the indirect competitors, and the accidental competitors.
Direct competitors are people who are doing exactly what you're doing.
A lot of the times, when you reverse engineer their keywords, you will get access to the same eyeballs that they are targeting.
Indirect competitors are within the same range of choices as your target customers or end users, but they are not offering the exact same thing as you are offering.
You have to be more careful in reverse-engineering these because there has to be clear enough communication of the value their content is offering.
Otherwise, you're just wasting your time targeting the indirect competitor keywords.
There are accidental competitors, and what I mean by "accidental" is that are fast-rising or in new niches that people haven't thought of as your competitors.
The key here is to look at the interest that they draw from your end users.
You have to be a little bit more creative here and think outside the box.
But if you're able to do this right, you can get a head start on your actual direct competitors, and this can mean all the difference.