Lotions are good for hairy or damaged areas of skin (such as weeping eczema – where pus is seeping out of damaged areas of skin). This is because lotions are thin and spread easily, but they're not very moisturising.
Ointments are good for very dry, thickened skin and night-time use as they're greasy, thick and very moisturising. Ointments are usually free of preservatives so are suitable for sensitive skin, but should not be used on weeping eczema.