Symmetric Encryption
Symmetric encryption means that the same key is used to encrypt and decrypt:
Public Key Infrastructure (PKI)
Asymmetric Encryption
To solve the problem of negotiating 100 keys, if you want to send something privately to 100 individuals/system, a public/private key is used. If a message is encrypted using public key then it can be decrypted only with private key, and on the same line, if a message is encrypted using a private key, then it can decrypted using only a public key (not even with a private key)