Force.com Sites allows developers to build and deploy public web sites and web applications
You can define custom domain names.
Force.com Sites are built with Visual force

Four Steps for Creating a Force.com Site

1) Register a domain name
2) Create a Force.com Site using the domain name
3) Assign Visualforce pages to the Site
4) Set up the security access permissions

Registering a domain name
Navigate to Setup | Develop | Sites and enter a domain name to use.
Example: yourDomainName-edition.na6.force.com

Creating a Site

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

A person account is an individual customer or consumer with whom you do a business. Example. A online shopper

Person accounts are not enabled by default in Salesforce.You need to contact to salesforce to enable it.
Where can I see Person Account?
Person Account can be seen when you create a new account ,You will see it in the form of record type.

Note: It works for Business to consumer model because here you are doing a business with a indivual consumer not with a company.

Accounts are customers, competitors, and partners.
Accounts can also be refered as Company.
Accounts have various fields to store information of your customers,competitors and partners for example name,address,phone number.
For every account you create you can have some related information such as opportunities,activities,cases,patners,contracts and notes.

Two different kinds of Accounts are there:
1) Person accounts - These Accounts represent the individual customers or consumers

What is Profie in Salesforce?

Proiles are a way you can group users together for easier administration.--(INTERVIEW)
A user can belong to one profile, administrators can change profile membership for a user.--(INTERVIEW)
A profile is a collection of settings and permissions that determine what a user can do in the platform.--(INTERVIEW)

Proiles control:
The objects the user can view, create, edit, and delete
The object fields the user can view and edit (more on that later!)
The tabs user can view in the app