The basics: the difference between kroketten and bitterballen. They are very similar, to start with, both are a soft roux filling with a crunchy outer shell that is formed during frying. The main difference is the size. Bitterballen are shaped like a ball whereas kroketten have a cylindrical shape. Both can have a variety of fillings, all characterized by a smooth soft inside and brown crunchy outside. The most common filling is a beef version, but nowadays both vegetarian as well as meat containing version are available using a whole range of different ingredients.
Since bitterballen are quite a bit smaller than kroketten they are more commonly served as a small snack during drinks for instance. Kroketten on the other hand, might be part of a lunch (eaten with bread) or dinner (eaten with fries). Both are often served with mustard!