What's the difference between IEnumerable<T> and List<T>?

IEnumerable<T> is an interface representing a sequence of items that can be enumerated one at a time using deferred execution, meaning the actual data is only produced when you iterate over it, which makes it memory efficient for large or streaming data sources. List<T> is a concrete class that implements IEnumerable<T> but stores all of its items in memory at once, and it also provides additional capabilities like adding, removing, sorting, and accessing items directly by index, which IEnumerable<T> alone does not support.

IEnumerable<int> numbers = GetNumbersLazily();
List<int> list = numbers.ToList();
list.Add(10);
var first = list[0];