Encryption and Decryption of a string using AES in C#

AES is a symmetric block cipher algorithm. A private key is used to encrypt and decrypt the message. Encryption of data can be done in various ways. Why Encryption is needed ? There are fields like DateOfBirth and Aadhar number or SSN which a user don't want to show to the others and he wants it to be encrypted before being saved to the database. We are using one master key to encrypt the data....