IEnumerable is an interface. It is part of system.collection namespace and it has one method, GetEnumerator. This is the method the foreach loop uses loop through a collection and returns an IEnumerator Interface.
An Enumerable class is part of Linq library and is a bunch of extension methods written for the IEnumerable interface. It has many extension methods like First, FirstorDefault, Any, Last, LastorDefault, Max, Sum, Average etc...