This is the single most important question to get you site the search engine rankings you are looking for. Many people make the mistake of chasing "Page rank" and as a result even get it. The problem with this strategy is that while you gain page rank (from links), search engines are confused about what you site is actually selling or even where it's located. As a result your site doesn't pull up under any searches that might lead a customer too you.
This is especially troublesome for photographers as many of our sites are flash sites and search engines are unable to interpret these images.
So how do search engines know what your site is about?
It's starts with the Title of you site. The title of a site is at the top of each browser when you open the page and it is one of you best tools!
A very common mistake is to have you Title say "Brandon Smith Photography" for example. What does this tell search engines? Well, that if someone is looking for Brandon Smith or Photography that this site might be relevant.
A photography title should specify what you do and where you are. For example "Brandon Smith Photograpy - Portland Oregon - Newborn Photography - Wedding Photography" you get the picture. :)
Next you want to fill in a site description. Your site description is what pulls up below your link in search engines. This should be something simple but with a lot of information about your business. Remember this will be visible so it should be a coherent statement.
Finally you want to fill out your "meta tags" This is the same as putting "tags" on your post. These should be the key words you want you site associated with.
So where do you find all of these "keywords"
1) Ask your friends, "if you were searching for a photographer, what would you google?" As professionals we use words or terms that most people wouldn't think of or search. A good example is "photojournalistic", most of your customers don't really know what that means.
2) Find out what your competitors our using! Here is a site that show you what your competitors are using in these categories.
Make those corrections and your on your way to the search results you want.