What is Aadhaar?
Aadhaar is a 12 digit individual identification number issued by the Unique Identification Authority of India on behalf of the Government of India.
This number will serve as a proof of identity and address, anywhere in India.
Any individual, irrespective of age and gender, who is a resident in India and satisfies the verification process laid down by the UIDAI can enrol for Aadhaar.
Each individual needs to enroll only once which is free of cost.
Each Aadhaar number will be unique to an individual and will remain valid for life. Aadhaar number will help you provide access to services like banking, mobile phone connections and other Govt and Non-Govt services in due course.
Some other information about Aadhaar:
Aadhaar will be:
Easily verifiable in an online, cost-effective way
Unique and robust enough to eliminate the large number of duplicate and fake identities in government and private databases
A random number generated, devoid of any classification based on caste, creed, religion and geography
Click Here for the official website
Click Here for How to Enrol
Click Here for Where to Enrol
Click Here for the Enrolment Form
Consumer Resources 
If you have applied and not received your aadhaar card, you can download it directly from http://eaadhaar.uidai.gov.in/
I think Aadhar is a type of UID number which identify every people in the country. Because if govt launch any scheme than UID person directly get that benefits. Which are really important thing of UID number.