Search engines rank sites based on a determined relevancy to the user's search query. Since search results consist of a list of web pages the more relevant a page is to a specifically focused topic the better chances it has to be returned for a given query. So for example, if a user is looking for "dog pics" then you could focus a web page by building it with the following attributes:

File Name: Dog-Pics.html
Page Title: Dog Pics
Meta Description: Dog Pics
Meta Keywords: Dog Pic, Dog Pics, Dog Pictures
Page Header: Dog Pics
Page content: Use the phrase “dog pics” 3 times for every 300 to 500 words.
Alt tags (image tags): use phrase dog pics where appropriate

A good way to determine what keywords to focus on per page is to research what words or phrases your clients search for online. Some good tools for this are the following:

7Search Keyword Tool:
http://conversion.7search.com/scripts/advertisertools/keywordsuggestion.aspx
WordTracker: http://www.wordtracker.com
Overture Keyword Selector Tool: http://inventory.overture.com