A true italic is an angled typeface designed to accompany its roman counterpart. It is designed with its own features and character widths, and is often quite calligraphic in appearance. An oblique is usually a slanted version of the roman face, often with very little change to the design of the letter shape. Of course, as with all things, there are exceptions to the rule: Helvetica and Optima are two examples where the oblique is actually an italic. Programs like InDesign make it possible to slant any typeface, which can come in handy if the type family does not contain an italic weight, but where an italic has been drawn, it is usually much more complementary to the roman face it is matched with.