What are Contacts in salesforce

Contact Object contains the detail of people that are associated with your business accounts.
Various kinds of information can be stored, such as phone numbers, addresses, titles, and roles in a deal

Social Contacts : Its let you to view your social network contacts directly in salesforce . Example LinkedIn®,Twitter™,Facebook®.
Social Contacts is secure and private
Salesforce uses the trusted OAuth protocol to sign in social networks
You can't import or store social information in contact records
Your view of a contact's social profile is not shared with anyone else in your organization

When you click on contact tab you see View Section, recent contacts

Note: If you change the Account Name field: Cases and opportunities associated with the contact remain associated with the previous account
New cases and opportunities created for the contact are associated with the new account