You can book a tour up to 30 days in advance.
The Discover the Magic tour, from what I understand, is like a little scavenger hunt around the park. You get clues and wind up in a specific spot that will have a little "prize"
The Walk in Walt's Footsteps is more of a normal tour, and it includes lunch. You learn about Walt, the park, etc.
I believe the Welcome to
Disneyland tour is no longer offered.
Some tours include going on rides without waiting in line, some include a meal or snack, etc. For example, on the Christmas tour, we got a gingerbread cookie and cocoa in a keepsake mug, fudge sample, a pin, and we got to get on IASW, HMH, and the train with no wait.