A .torrent file holds the release metadata — file names, total size, piece size, hashes — plus tracker addresses. It carries no content itself; it only describes what to download and from whom.
A magnet link solves the main weakness of the .torrent file: it contains just the info hash and fetches metadata straight from the DHT network. Downloading starts without an intermediate file and without depending on a tracker.
In practice magnet links are more convenient: they are shorter, survive tracker shutdowns and work in every modern client.
