If you spend any time at all examining websites on the internet, you'll notice a trend where people target single keyword search terms such as "dogs" or "weddings" or "jewelry". These are a small segment of all available keywords, however the numbers are enough that a discussion is in order about attempting to rank for such terms.
Getting down to the nitty gritty of the matter, it makes very little sense to target single keyword search terms - the majority of the time. There are always exceptions, of course, but let's discuss the 3 biggest reasons why it usually does not make sense - or money!:
1. Untargeted Traffic
Generally, it's next to impossible to know exactly what an internet searcher is looking for when they type in a single search word. Let me demonstrate. An individual looking for "dogs" might be looking to buy a dog, or maybe they're searching to find information about how to train dogs, or maybe they just want to see photos of the various breeds of dogs. There really isn't a way to know for sure based on a single keyword search term.
2. Low Click Through Conversion Rates
Since the traffic is so broad and untargeted - and because the results are so large, and because we cannot know for sure exactly what a searcher is after, conversion rates coming from single keyword searches are naturally going to be very low. Staying with our previous scenario, if someone is searching for "dog training" we'll know precisely what keyword phrase to have on the page that we want to rank for. The search term is much more defined. Now we know exactly what the person doing the searching wants, and consequently, we can focus in on the desire and fulfill the need the searcher has.
If you trying to get AdSense to work for you, or some other pay-per-click income program, a low "conversion rate" means a low click-through rate. Furthermore, context created ads such as AdSense, will have a harder time knowing what ads to serve on pages that are very general in nature. However a page like "dog training", will help AdSense deliver precise ads to the page that is optimized for the term "dog training".
With a single keyword search term it is "hit or miss" as to whether ot not what you have on the page appeals to any of the visitors locating your page through broad search terms. Of course there is always the option of providing a lot of choices on the page (dogs for sale, dog pictures, dog beds, etc.) but keep in mind that too many choices isn't helpful for improving conversion rates. This is simply because a lot of choices on a page causes your web visitors to be overwhelmed and thus less interested to act in the way you wish.
Also bear in mind that people searching for a single keyword is much less likely to know what they want when they begin their search. Often they are starting out with an idea that they haven't narrowed down, or they might just be researching. In any event, If they aren't sure of what they're looking for, it's unlikely they'll take the action desired, and hence, this leads to lower conversion rates.
3. Too Much Work For Too Little Return
So why all the battle over single keyword terms? After all, in spite of the lower conversion rates for single keyword terms, they present a nearly insurmountable effort to get good search engine placement in virtually all situations. The reason for this is due simply to the fact that many large companies with deep-pockets view high traffic volume as a "thing" to strive for and consequently, just as important as making sales from that traffic.
Very large companies are looking to create a public identity as a way to brand themselves about as much as they are striving to actually sell something to potential customers first visiting their web site. Often their primary objective is get their name out in front of as many people as they can. They theorize that if they do this on a consistent basis, there will come a time when the customer realize a need, and then their name will pop into the customer's head. It's basic marketing psychology, and it has been proven to work well.
However smaller marketers can't afford to wait years before they see a return on their investment through name branding. For them it's important to have a good conversion rate from the beginning in order to warrant the constant effort and expense it takes to keep up their search engine rankings. If you're onethese types of marketers, then ranking for single-term keywords probably isn't the direction you should take.
There are Exceptions
There are, of course, a few exceptions. Some single-term keywords, by their nature, are fairly narrow to start off with, and consequently such keywords will be more likely to be worth getting a rank for.
For instance, if someone were searching for the acronym "WEP" (Wireless Encryption Technology), its quite likely they are curious about internet security, so it's not difficult to design a page around that need if you rank for that term. There are quite a few other terms that are also narrowly defined enough to be fairly targeted, albeit they're few in number.
Furthermore, if your website relies on a CPM advertising model (where you're paid by the number of ad impressions, rather than the number of ad clicks), getting a good ranking for a single term keyword can be quite profitable. This is due to the fact that those same companies that want to build brand awareness are also happy to expend capital into advertising for that same reasons. They aren't concerned with the number of clicks, but rather the number of people viewing such that they get the company's logo indelibly burned in the minds of the website visitor.
The Bottom line
When you're beginning to decide what keywords you want to rank for, always bear in mind that, unless you're looking to brand your site or to earn from a CPM model, single keyword terms are usually not worth the time and the effort. Instead, focus on subcategories and find products or services that allow you to create a more targeted keyword phrase.