What's the difference between IOrderedQueryable and IQueryable?

I have this:

    var points = from p in ContextDB.Points
                 orderby p.PointInTime descending
                 where p.InstanceID == instanceId
                 && p.ParentPointID == null
                 && p.PointTypeID == currentPointTypeID
                 select p;

and this:

    var points = from p in ContextDB.Points
                 where p.InstanceID == instanceId
                 && p.ParentPointID == null
                 && p.PointTypeID == currentPointTypeID
                 orderby p.PointInTime descending
                 select p;

While I understand the use of both (and one generates an error later) I don't understand how they are different.

I did see questions like this elsewhere on STO, but I've not garnered what the answer to this question is, I'm afraid.