Tuples are immutable, lists are mutable. (That is, once a tuple is created, its not possible to add or replace elements in it.)
Because a dict only allows immutable objects as keys, a list cannot be used as a key in a dict. (In order for an object to be used as a key in a dict, the object needs to be hashable, that is: hash(obj) needs to be defined).
Typically, tuples are used in a similar fashion like a C struct with a predefined number of elements and types.
Lists on the other hand are often used like arrays with a varying number of elements.