How can you encrypt and decrypt a message in cryptography

There are two main types of encryption - symmetric and asymmetric. 

Using symmetric encryption, you use a single secret key that is used to both encrypt and decrypt the data. This form of encryption has been used way before computers! Without this key it is very hard to decrypt a message. However, the issue with this is that if you want to send an encrypted message, you also have to send a key to them, and if your somebody unintended intercepts the key, they can read the message.

What's the solution? Asymmetric cryptography or public key encryption. Asymmetric cryptography uses encryption that splits the key into two smaller keys. One of the keys is public and one is kept private. A good metaphor for understanding this is the public key is the lock, and the private key is the key. You can encrypt a message to send to somebody with their public key, locking it, which they can then decrypt, unlock, with their private key. And vice versa. So with this kind of encryption it's very difficult to decrypt a message for anybody but the owner as it's much harder to steal when it doesn't need to leave their computer, but easy to encrypt messages to send to them