Aadhaar UID Number.

Aadhaar is a 12-digit unique number which the Unique Identification Authority of India (UIDAI) will issue for all residents in India. The number will be store basic demographics and biometric information – photograph, ten fingerprints and iris – of each individual in a central database.

Here’s the wiki for Aadhar UID – http://uidnumber.org/aadhar/wiki/Main_Page

It has got some useful information. FAQ  – http://uidnumber.org/aadhar/wiki/Frequently_asked_questions

Book your slot from this website http://uid.karvy.com/BookmySlot/

Three easy steps –

1) Enter your personal details

2) Choose your nearest center

3) Select your preferred date and time of aapointment

I have not yet taken the appointment. Planning to take it soon.