Traffic is another word for the visits to your site but it is usually used when referring to groups of visits. The source of a visit is the specific place that sent the visit to your site. Sources include Google.in, Google.com, Twitter.com, paid advertising, display ads, bookmarks people have in their browser and loads more. These are grouped in to the following main Mediums:
1) Organic – Non paid search traffic
2) Referral – a visit through a link to your site from another site
3) (none) – this is usually all visits that come direct to your site, through users bookmarks or if the user types the address in to the browsers address bar manually. It will also include any visits where data cannot be passed on from the entry source.
4) CPC – This stands for Cost per Click and refers to paid advertising, Google AdWords visits can automatically be tracked within this Medium and you can manually set up any other advertising to also be grouped under this Medium.