A swarm is a group of peers that are sharing the same torrent.
What Is a Swarm?
Swarm is the word used to describe a group of peers sharing a torrent. There may be multiple swarms for one torrent. In some cases, peers create their own swarm. It is also possible to share a file yourself, but this is called “seeding.”
A peer is an end-user or client who uses the BitTorrent protocol on their computer to download and upload torrents. Peers exchange information about torrents with each other over the internet for the purpose of sharing files via BitTorrent.
The BitTorrent protocol allows only one swarm to share a file at any given time. You can think of a swarm as being similar to a conference call; everyone in the swarm shares the same conversation, and only one conversation can exist at any given time. In this case, the conversation is about the particular file being shared.