Rupert Atkinson

Jun is right on the mark, except that most schools adopt an either / or approach. They either use irimi & tenkan, or omote & ura, but not both. Jun's description is actually better - more useful.

Also, for myself, I have found it very useful to think of irimi as reversing uke's energy back upon uke, and to think of tenkan as allowing it to continue on, with a little extra added for good measure. I find this helps my Aiki more - thinking of 'movement' rather than 'position'.