You can invest huge amounts of money and time in promoting your web site, but you need to know where your traffic and sales are coming from to make it effective.
Many webmasters just place a counter onto their web site.
All this does is tell you how many people have seen your site. It doesn't tell you where they came from, what browsers they were using, what keywords they searched for at a search engine, etc. To be able to effectively market your web site and to receive the greatest amount of results, you must track your visitors. There are many ways this can be done. There are programs available like WebTrends, Web Site Reporter, etc.