What is meant by Public Key and Private Key?

A PKI cryptographic system occurs through the use of extremely long prime numbers, called keys. Two keys are involved - a private key, which only you have access to, and a public key, which can be accessed by anyone. The two keys work together, so a message scrambled with the private key can only be unscrambled with the public key and vice versa. When someone wants to send you a secure message , he/she uses your public key to encrypt the message. You then use you private key to decrypt it.
An important element to the public key system is that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key if you know the public key. When you get your certificate from Etisalat you need to keep your private key (.pfx extension) secure and safe with you, whereby you can share your Public Key (.cer extension) to anyone including Dubai Customs.

What is PKI (public key infrastructure)?

A PKI (public key infrastructure) enables users of the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates.