Different materials can absorb, reflect, scatter, or even bend light. A lens is simply a curved piece of glass that takes the beams of light bouncing off an object and redirects them to a focal point, forming an image that looks exactly like the object in front of the lens. Because light travels faster through air than it does through glass, the lens slows it down.
When light enters a piece of glass at an angle, one part of the wave will reach the glass before another and will start slowing down first. Depending on the angle at which it enters, it bends in one particular direction. It bends again when it exits the glass, as parts of the light wave enter the air and speed up before other parts. This is called refraction.
In a concave lens, one or both sides of the glass curves inward, causing light to spread out, like a wave. In a convex lens, one or both sides of the glass curves out, so that rays of light passing through will bend toward the center of the lens to focus it to a single point.
In a microscope, a concave lens gathers light from the specimen. Because the sample is small and nearby, very little light is needed for the task, so the concave lens is small and curved, giving it a shorter focal length on either side. This brings the specimen into focus at a short distance with the microscope's tube. The image is then magnified by a second, convex lens as it is brought to the eye. The tube holds the eyepiece at the proper distance from the objective lens and blocks out stray light. And of course, some sort of light source is needed to illuminate the specimen.
Build your own microscope!
All you need are two magnifying glasses and a sheet of printed paper. Hold one magnifying glass a short distance above the paper so that the print looks a bit larger. Then place the second magnifying glass between your eye and the first magnifying glass. Move the second glass up or down until the print comes into sharp focus. It will appear even larger than it does using just one magnifying glass.