Lead is nothing but information about an organization or an individual person who is interested in your product.
Lead Source is the primary source from which the lead is generated.
Sample Lead sources are
2. Phone Call
6. Business Card
7. Customer Service
10. Trade Show
11. Employee Referral
12. Social Meda(Facebook, Google Plus, etc…)
13. Search Engine.
Lead source list changes from one organization to another organization based on the type of business and services they provide.