An eyepiece, or ocular lens, is basically the lens through which one views in optical devices such as telescopes and microscopes. They are attached to such instruments, and used to magnify whatever object is pointed at.
There are basically two lenses in an eyepiece: one is the field lens, which is where light passes through and images of the object are collected; and the eye lens where the observer looks through and sees the image. The viewer then can magnify or sharpen whatever he is seeing with the use of the aperture. The number of magnification depends on the focal length, the distance of the field lens to the eye lens, of an object.
There are also different kinds of eyepieces, ranging from the simpler to complex ones. Some have low field of view, while others are so powerful they are best for lunar or planetary observations. Huygenian eyepiece uses two convex lenses, which is simple. The Kellner eyepiece has two convex lenses but their flat surface facing outwards, plus an achromat for the eye lens. Another one is the orthoscopic eyepiece, which consists of three-cemented field lens with a single convex eye lens. There are also other types of eyepiece.