Lenses
A: Positive, Converging lens, Biconvex or plano-convex

Rays focus a real image at a point on the opposite side of the lens from the source.

F is called the Principal Focus; f = focal length

B: Diverging lens, Biconcave or plano-concave

Rays diverge in the image space and do not form an image. Instead, the eye "sees" a virtual image in the Object space.

Conjugate Planes
In a perfect lens system, all points along an imaginary plane (in Object Space) will be focused to a corresponding plane in Image Space. These two focused planes are conjugate planes. In confocal microscopy, the pinhole is places in a plane conjugate to the objective sample plane.