IB_Cursor is unidirectiona, unbuffered dataset. It's fast and needs very limited resources on the client side. So it's fabolous for reports or for walking throurgh a (small) table, like when you have to fill a combobox.
Query is the most used since it's buffered and "full option" dataset access. You know, database servers are unidirectional (ok, often new ones are not, but that's the C/S paradigm), so if you want to walk back you must have data on the client side (into buffers). Of course this brings out syncro and other problems... so welcome in the Client Server world :))

Marco Menardi