Por si las dudas, hermoso.
IEnumerable, ICollection, and IList (generally, any type with an I prefix) are just interfaces. They let you expose what your class will do, but unlike if you inherit a class, interfaces do not provide you a default implementation of any of the things they say you must do.
As far as choosing which interface, here’s a quick guide:
An IList is an ICollection that can be accessed by index.
An ICollection is an IEnumerable with easy access to things like Add, Remove, and Count.
An IEnumerable is anything that can be enumerated, even if the list of those things doesn’t exist until you enumerate it.
Some classes that you might want to extend (or keep as a private field that runs most of the logic) for your collection are List<T>, Collection<T>, (which implements IList<T>, but with easier access to overriding implementation, see Collection<T> versus List<T> what should you use on your interfaces? for the big differences between these two) ObservableCollection<T>, or collections that are not lists, like Dictionary<T, U> and HashSet<T>. For more info on any of these, look up the MSDN documentation on the class.