I need a tool that determines which keywords a domain ranks for.

Does any tool exist that shows which keywords "domain.com" may rank for in the search engines?

 

Google's webmaster tools does show the relevancy of particular keywords associated with a particular site, you might want to try it out. There are others as well but I'm not particularly sure which are best.

 

If you know how to do a keyword research and moreover if you know how to research your competitor there is no need for a tool to figure it out what keywords your competitor is ranking for....

Also On-Page SEO Factors can give you a good idea too for the keywords they are trying to target.

 

I don't know though. Can someone teach me or help me out with this please?

 

Teach yourself my friend. Nothing in the online world is hard to learn...The hard thing is to keep working to become somebody and make good money online.

Here you go.

Download the Market Samurai from http://www.noblesamurai.com/

and then learn from these videos --> http://www.noblesamurai.com/dojo/marketsamurai/

You will learn a lot just from there...

 

I get this info for free from Alexa.com -- put in any URL and they'll tell you a 'world' of keyword density % and other info for the keywords they're most using.

 

SEMRush provides a list of all the keywords that a particular site ranks in the top 20 for.

 

If your website is listed on alexa you can see the keywords for which your website is ranking high.

 

That is your answer. Nothing left to say! Other than what SEMRush doesn't find, you can check for manually. Typically, SEMRush will find more than you can right off the bat.