There are literally thousands of different formulations of glass in the world. Each formulation has its own very specific optical properties and consequently its own designation. BK-7 and Bak-4 are two such designations of glass formulations used in the prisms of binoculars, spotting scopes, and monoculars.
BK-7 (as well as the very similar K9) refers to a type of borosilicate glass that has long been and still is widely used across many different optical product brands and models. BK-7 and K9 are lower in cost than BaK-4 and therefore are often found in lower-priced products than BaK-4.
BaK4 refers to a light barium crown glass that, as it is much more expensive than BK-7 or K9 glass, is generally found in mid-to-high-end price ranges optical instruments. For binoculars, spotting scopes, and monoculars, BaK-4 is generally preferable to BK-7 or K9 as it has a higher refractive index and when used in a well-designed overall optical system, provides higher levels of image quality than would a prism assembly of the same type made with BK-7 or K9 glass. It is often written that the exit pupil of a BK-7 or K9 prism is not perfectly round which causes clarity issues on the edges of the image, while a BaK-4 is perfectly round. This is sometimes true but should not be trusted absolutely.