Seeds means just that. Users that have 100% of the torrent & are only downloading. Peers/leechers are those at less than 100% who you are d/ling from & uploading to.
With 0(21) in the seeds column this means there are 21 seeders in the connection pool , but you're not connected directly to any. Because of the way BT works it can still mean that there's 100% of the torrentavailable directly to you without any of the peers having the 100%.
Leechers on the other hand or like seeders but they don't upload anything anymore, think of them as parasites who only get what they need and don't contribute to the community