For actually getting torrents loaded into the client software, there’s basically no difference. They both contain the information needed to start downloading the content.

A torrent file just has the information needed to start the download, kind of like a text file.

A magnet link is similar to any other link, like http://example.com/SomeFile.torrent. They’re a bit more complicated because the website example.com might remove or change the location of SomeFile.torrent and then any of the existing links won’t work. Magnet links aren’t affected by that sort of thing. I say this with the caveat that I honestly don’t really know or understand them fully because, like I first said, it doesn’t really matter.