In theatre we have many types of lanterns and they all perform different tasks. I will explain simply what each does and in a later article I will study them in depth.
A profile spot consists of a lens, reflector and a lamp. The lens is convex in shape and you can project a hard edge beam of light. This type of light is focused by moving the lens closer or further away from the lamp. If a lens is placed between the focus lens and the lamp, you can then vary the size of the beam. This is similar to the zoom function of an camera. These type of profiles are often called zoom spots. These lights are mainly used from the front and also as specials where you need tight control of the focus.
Below is an example of a profile lantern.
A fresnel gives a soft edged beam of light, and this is due to the lens. If you look closely at the lens you will notice that it is made up of concentric rings, this softens the light coming out of it. The only adjustment that you have is varying the size of the beam. This is achieved by moving the lamp and reflector closer or further away from the lens. These are used on stage to light the acting areas.
A flood basically does what its name implies. It floods an area with light. You have no real control over the focus. You can sort shape the beam with barndoors. The main use of these lights is to light scenery.