The truth is that any torrent client can do the basic stuff (download, that is), but the differences (especially on Mac) are usability and the required resources. Talking about resources, I choose to ignore torrent clients like Vuze (Aureus), because they are Java-based.
There are currently a lot of options for Mac OS X, but this is kind of a new situation, "historically speaking." Not so many years ago, uTorrent (the most popular Windows torrent client), did not have a Mac version.
The top 3 torrent clients for Mac
The first and kind of default torrent app in the Mac World is Transmission. It was good and simple to use even when the "new uTorrent" was a revelation on the Windows front. Transmission is free, open source, and the default settings work very well.
It's "Plug & Play," which means that it will automatically open "magnet links" and .torrent files; the default saving folder for torrents is "Downloads." Anyone can easily create a new torrent file to share. The app size is just 11.2 MB.
The second option is new to the Mac World. Its name is Deluge (initially called gTorrent), but it doesn't seem to be as actively developed as Transmission or uTorrent. It uses GTK+, so the interface looks more like Linux Gnome. It's a complex client and the usability is not really great.
The important thing about it worth mentioning is its support for Plugins. This can prove very useful for advanced users, but the app size is 71.4 MB.
The third option is uTorrent. It should be the second option, actually, if only it weren’t for its Adware problem. This means it will display ads and commercial messages in the interface. The ads can be removed if you pay 5$/year, however.
The interface appears to be a combination between uTorrent for Windows and Transmission. For a Windows user, it will seem very familiar, but it was clearly not designed for Mac from the start. Its app size sits at 9.1 MB.
P.S. It doesn't matter what torrent client you actually use, the basic instructions and problems are the same for heavy users nonetheless, so I invite you to read the uTorrent Optimization Guide for further info.