Actually a private key is nothing more than a 256bit random sequence (e.g. 010110...1001)
The words that you refer to, derive from this 256bit number, if you add 8 bits more (checksum).
The seed is a representation of that number sequence in human readable and understandable form. It's much easier for us to look at and understand individual words and their meanings compared to a long sequence of (to us) illogical characters. That's why it's recommended to make physical backups of those seed words instead of the long number sequences where a mistake can happen.