In fact my great grandfather used to make telescopes in his optical business. The most simple telescope is called Refracting. It has a larger "objective" lens at the front, convex like you would use in a magnifying glass, behind that is a smaller lense; the "eyepiece lense" which is concaved (curves inwards). The shape of the front lense gathers, bends and concentrates light that then passes through the back lense which (due to its shape) makes things look clearer, bigger and so nearer.