When light rays reflected or refracted from mirror or lense ,they passthrough focus. Some rays which are parallel to principle axis ,after refelection or refrection ,they return again either paralle to principle axis or straight . So they form image. If an image can be formed on the screen or can be taken on screen ,is called real image. Those cannot be formed on screen ,known as virtual image.Image formed in plane mirror is vitual .where as some iamages formed by lense (concave or concave) or mirror (concave mirror or convex mirror ) are real.