In Symmetric Cryptography there is a single key used to both encrypt and decrypt the data being transferred. When the sender encrypts their data it is locked with a key to keep the data unreadable and out of the hands of the general public. The key is then transferred to the destination it is intended for and they use this key to unlock the data and decrypt it making it readable again.
Symmetric Cryptography is fast and efficient taking little to no time to encrypt and decrypt.
Having only one key is a security risk. If the key is lost or stolen, an outside source now has access to your messages even if they are encrypted.
Asymmetric Cryptography is also known as public key encryption. Within Asymmetric Cryptography two keys are used. The first key is a public key that the general public has access to. Everyone can see this key and when you want to send a message to another person, you take their public key and encrypt your message with it. The second key is the private key. This key is never shared and held onto by only the reciever. When they receive a message encrypted by their public key found online, they use their private key to decrypt the message and see the original content sent by the source.
Asymmetric Cryptography is much more secure. There is no need to transfer keys so it keeps your data secure so that only the people who are intended to receive the information actually get it.
Due to the extra steps of using 2 keys the encryption process is over a million times slower than Symmetric encryption.