* "Deer" is a general term: It refers to any member of the family Cervidae, which includes a wide variety of animals like elk, moose, caribou, and reindeer.
* "White-tailed deer" is a specific species: It's a particular type of deer (scientific name: *Odocoileus virginianus*) commonly found in North America.
So, all white-tailed deer are deer, but not all deer are white-tailed deer.
Think of it like this:
* Deer: Like the category "fruit".
* White-tailed deer: Like the specific fruit "apple".