google and ebay are made up words that had little to do with their respective category until they became the dominant player.  Much like xerox or coke, their name eventually became synonymous with their product.

cars.com and weather.com are generic names that have taken on a brand of their own, but are very descriptive of the category in which they do business.

Plenty of sites do some work in between.  My ad agency name was created by removing the “ad” from advertising it and replacing it with ROI, (ROIvertising.com).  Since I made it up, it cost me $10/year.  This domain is a cute way to say escalate phonetically.  These take the approach of building a brand, even for small businesses.  After the initial purchase, all of the attributes will come from my investments into building a brand.

Others take the approach of using very descriptive phrases.  These are domains that you see and instantly have a guess to what they will deliver.  A couple of random selections:  I’m pretty sure that I’ll get apple rumors at macrumors.com and can buy lingerie through simplelingerie.com.  They are pretty good to be descriptive, and most of the work is done up front.  They also have some advantage through search engine optimization.

Even though my personal preference lies in building a brand, for many businesses paying the premium for a direct, descriptive domain can be worth it.  Natural Search traffic is “free” traffic after the inital expenditure.  Its possible to model the value of a descriptive domain by looking at the cost per click for the type of search and estimating the traffic.  Do the math and figure out your best path.  The answer won’t be the same for everyone.