Toast 10 is primarily a disk burning utility, and a very competent one at that. Although OS X includes a built in disk burning utility, it's basic to say the least. Toast has all the burning functions you would ever need - Data, Audio/MP3 CDs, enhanced Audio (i.e. Audio and Video on the one disk), Video DVDs, Blu-ray disks (you get a free plug-in to allow you to author blu-ray and you can even burn HD content to a standard DVD and play it back on your blu-ray player; a great way to get the quality of HD, although you can only fit approx. 20 mins of content on a DVD). Toast also taps in to Apple's iLife media browser, allowing you to easily take your media from iLife and export it using Toast.
As well as all of these fairly standard features, Toast is also an excellent file converter for video and audio. Take a Windows Media video for instance, and it can easily convert it for playback on your Apple TV, iPhone/iPod, Blackberry or games console. You can even convert a whole batch of files at one time. One really interesting feature is that you can import a multi-cd audio book, and Toast will automatically convert it to a singe audio file with chapter markers for you to send to iTunes.
The interface is really straightforward - just drag and drop. I've been a user of toast for 5 years now, and this is a great evolution of the product. It seems very fast and stable (I had problems with the previous version frequently crashing); it converted a batch of 35 video files without a problem.
To summarise, this is an excellent utility for the Mac - a great compliment to iLife, and in my opinion an essential program.