What is fundamental difference between item.Axes.GetDescendants() and item.Axes.selectitems() in sitecore

item.Axes.GetDescendants() get all the descendants of the item using Sitecore API. It calls item.Children, and then for each child again child.Children recursively. And it adds all those items to an array.

item.Axes.Selectitems(string query) executes Sitecore query passed in the argument in the context of the current item.

So those 2 methods are completely different.