Web traffic, like highway traffic, generally comes from a number of different directions – or in this case sources.  Typical sources are Google, Yahoo, Bing, Referral sites, Direct sources, article directories, forums, social media sites, bookmarking sites, blogs and so on.  So your goal must be to determine what sources are more successful than others and where you should be putting your focus and financial investment.  The truth is the only way you can find this out is by tracking and measuring each traffic channel and analysing the results.