The major sources of traffic to your website may vary, but, typically, a mature website will have traffic from three major sources:
Search Engines: 80%
Millions of Internet users make use of search engines like Google and Yahoo! to find their products, services and information. If you lack an active ad campaign, up to 90% of your new visitors will come from search engines; therefore, proper search engine placement is essential.
Inbound Links: 15%
Inbound links from other websites provide up to 15% of your incoming traffic.