There is only one file type that uses the .TOAST extension:
Toast Disc Image
A TOAST file is a CD or DVD disc image created with Roxio Toast, a popular disc authoring program for Mac OS X. It is similar to a standard .ISO disc image, but contains specific Toast formatting information.
NOTE: Changing a Toast disc image's file extension from ".toast" to ".iso" may allow the file to be opened by Windows disc authoring programs as well as Apple's Disk Utility software. However, compatibility is not guaranteed.
Updated: January 16, 2016