what is symmetric and asymmetric key cryptography?

Symmetric  key cryptography ? 

Symmetric key cryptography is oldest method of encrypting the message.It is a simplest method that involves only one secret key.This secret key is a simple text like number,string,letter,etc.
A simple example of symmetric key encryption is that suppose I want to send " hello " to someone then to encrypt it I will replace every text of this message into third text like for 'h' I will replace it with 'k' and 'e' with 'h' . so..
our message is : ----   Hello
Encrypted message is :---khoor

so if the unauthorised person read the encrypted message the he is not able to understand it .So our message is always be secure from unauthorized acess.

Example :- RC4, FISH, Py, QUAD, SNOW etc.

Asymmetric key cryptography?

Asymmetric encryption is also called public key cryptography.This is new technique instead of symmetric key cryptography.Asymmetric key cryptography uses two keys to encrypt the message .one is publicly available and another is private .

if someone wants to send message to me then I have private key and public key has them who wants to send message.

Example : RSA,SSH,etc.