Prism Lenses with a prism component shift an image horizontally or vertically to compensate for differences in alignment between the eyes. In general the dioptric strength of a lens is the inverse of its focal length in meters. The same is true for a diverging lens but we're talking here about a virtual image instead of a real image. The first number in a prescription is the spherical correction, i.e., the amount of power needed to correct either myopia or hyperopia. The second number is the cylindrical correction, i.e., the power needed to correct astigmatism.
Cylinder involves two numbers, a power and a meridian in degrees.
Cylinder power is at its maximum 90 degrees off the stated meridian and zero 180 degrees off that. Nearly all lenses used in spectacles are convex on the front and concave on the back. The total prescription of the lens is equal to the sum of the front and back powers. And any lens has two such boundaries, one in front and one in back. Notice that this implies that a plus lens must be more convex on the front than concave on the back, while the opposite is true for minus lenses.