One of the most difficult aspects of running an online business is getting visitors to your site. The best way to get visitors is to have them find your site through a search engine, unfortunately these are the most difficult visitors to get since your competitors are all trying to do the same thing. Search engine optimization is the way that you can get your site to rank near the top of the search results however most people are confused by how to do this. There are a number of factors that go into SEO but by far the most important is links.
Most people would tell you that the most important factor when it comes to SEO is the number of links that you have coming into your site. Back links are how search engines determine the popularity and the relevance of your site. A link to your site is basically a vote that says that owner of the other site likes your site. The reason that this is important is because the search engines need something to base their search results on and the primary way they do it is with links. Obviously the search engines can't have somebody reading every single page on the internet to determine which one is the best and should be ranked first so they need another way to do it. They have made the not unreasonable decision, that the number of links is a good indicator of the quality of the website, the theory being that the better the site the more people will link to it.
The number of links that you have coming into your site is just a part of the equation, the other part is where the links are coming from. Links coming from sites that are on topics that are related to yours will have more value than links from unrelated sites. This is part of how the search engines determine relevance, a site with links from a whole bunch of related sites must be highly relevant to that topic. The other big factor in determining the value of the link is the authority that the linking site has, Google does this through page rank. Authority is by and large determined by the number of links going into the site that is linking to your site, a link from a site that itself has a lot of links is more valuable than a link from a site that has no links.
An often overlooked factor when linking is the text used in the link, ideally you want a link with anchored keyword text. That means that the link itself should include the keyword that you are trying to rank for. This is another way that the search engines determine relevance, they use the anchor text to help them decide what your site is about. Obviously you can't control the text used by another site when they link to your site, although it never hurts to contact them and ask if they will change the link to use your keyword. One way to help ensure that your keyword shows up in the anchor text, at least most of the time, is to use the keyword in the title of your website, most webmasters will link to you using that title.