symmetric/asymmetric encryption

symmetric - uses the same key, generally a session key

is the primary way we encrypt data

how to exchange session key? 
in-band - key goes with data - bad way
ephemeral key (temporary), provides perfect forward secrecy, uses asymmetric encryption (key-pair, public to encrypt, private to decrypt)
problem with asymmetric is slow, so usually it is used to send secure session key and then the sesson uses symmetric encryption