Peer to Peer Network Discovery

Please forgive me as I continue to use this blog to document my journey into peer to peer networking over the internet. This journey was sparked by the reportedly decentralized nature of the Bitcoin protocol.
First of all, what is P2P?
How does a new node know the addresses of peer nodes on a peer to peer network?
So far it looks like that new node must first connect to a server that has an explicit list of peer nodes

Peer discovery is the process of discovering other peers in a network. BitTorrent protocol use tracker servers to maintain a list of peers in a swarm. So, when you start to download a file using a torrent, the client can immediately get a list of peers from the tracker information in the torrent file.

- https://medium.com/tqtezos/peer-to-peer-networking-in-tezos-942b123fa6a
What is a swarm?