When you first set up your account and sign in, Analytics shows a Dashboard module containing a pie chart of all of your Traffic Sources. The pie chart is a breakdown of the different kinds of ways that traffic reaches your site. To explore your Traffic Sources reports in more detail, you can click "View reports" in the Dashboard module, or click on Traffic Sources in the left-hand navigation.
- Referring Sites: shows visits from people who clicked to your site from another site. Having a strong network of relevant referring links can be a great source of traffic, as well as useful information for your visitors. You might see jumps in this statistic if you do something with your site that gets picked up by the blogosphere. As news spreads and more blogs link back to the original source (your site), you'll notice more visits from this category.
- Direct Traffic: shows visits from people who clicked a bookmark to come to your site or who typed your site URL directly into their browser. Improvements in Direct Traffic numbers may mean that you are generating a lot of loyal visitors who are adding you to their favorite pages.
- Search Engines: shows visits from people who clicked to your site from a search engine result page regardless of whether it is Google.com or another search engine. If you're doing search campaigns, you'd look to these numbers for signs of improvement. This counts visits from nearly all major search engines including international ones, and more are frequently added to Analytics' index as they gain popularity.
- Other: is a catch-all category that represents any source not falling into the above categories. If you're new to Analytics, this is usually insignificant.
Source:→ AdWords Blog