You should always add your most important keywords to your titles. Some search engines will also seek in the first few lines a words written in a bigger font, like a headline. Frequency is another important factor most of the search engines look for. They will analyze how often keywords are repeated, how near they are of each other. Search engines give higher relevancy for a high keywords frequency, but they may also give you a lower ranking of the keywords density is too high, which they consider as spam.
Meta tags are an important factor on a majority of spiders.
Meta tags allow you to write the description of your site, choose keywords and it as other non-search engines related utilities. Some search engines ignore meta tags, other give slight boost when you have them, others give you really bad ranking if you do not have meta tags. These search engine will give a higher ranking to a site which is linked by many other site. They feel a popular site is often a good site. Google is going a step further, it gives higher ranking to website linked by other site containing the search query.