Light can be broken down into 3 particular qualities that you will need to pay attention to:
Light Quality can most easily be described with 2 adjectives; hard or soft.
Hard light tends to produce shadows with well defined, sharp edges. The transition between light and shadow is abrupt.
Soft light produces a smoother gradation between light and shadow, so that the shadow edges look feathered or soft for a smooth transition.
Quantity refers to intensity or how much light there is. An even exposure will generally have shadow, highlight, and well exposed midtones. Images with lots of light and little midtone or shadow are referred to as "high key." Images that are dark and have less light are called, "low key."
Direction refers to where the light is coming from in relation to your subject. Light direction is key for creating shape.
All of these qualities can be manipulated (and/or planned, if shooting in natural light) to produce whatever effect the photographer desires. Instruments used to manipulate the quality of light are known as "light modifiers" or simply "modifiers."
The job of a modifier is to manipulate the light that is available, whether natural or artificial, to produce a desired effect.
There are 3 main ways light is modified; by reflection (which can modify all 3 light qualities) diffusion (which tends to modify quality and quantity) and flagging (which "eats" light and affects quantity)
Reflection is the most common way to manipulate light and one of the easiest since reflectors can be found almost everywhere. Certain kinds of reflectors can even serve to diffuse (larger sources such as a wall or the side of a building and white colored material) or intensify (silver reflectors or "hard" surfaces that have highly reflective properties like mirrors) available light rather that just redirecting it.
Diffusion refers to breaking the light up with the main goal of diminishing it's quantity and often softening it. Sheer window curtains are good example. They diminish the amount of light coming through the window and break the light up so that it's softer as well.
Flagging is a term used to describe blocking the light and is most often done with something black, such as a piece of foam core board or a black material. The purpose is to block the light or reduce it. Light can be flagged from a subject or even from the camera itself.
When you are preparing for a shoot, it's best to visualize what kind of light you want and plan accordingly. If you are shooting in natural light then you need to know exactly what kind of light you want. Morning and evening provides diffuse directional light with morning light being cooler and evening light being warmer. Mid day produces hard light that comes from high angles and is not generally considered flattering for most people without some kind of modification.
If you use artificial light, you have a few more options and aren't quite as hampered by the position of the sun.
Your job this week is to start paying attention to light. Look around you and see where the light is coming from? What angle is it falling at? Is it hard or soft? Is there a lot of light, or just a little? How is it falling on the faces of the people around you? Is it flattering?
Take 1 day out and shoot all 3 qualities. It doesn't need to be a person as long as the light quality is evident. Post 3 photos in the comments below that show all 3 light qualities and explain what you are showing in each photo; quality (hard or soft) quantity (a lot of light or just a little) and direction (light that is coming from an angle)
Also, watch this great tutorial from B&H photo with Neil van Niekerk. He get's much more in depth with light than I can do here.
this course by Tony Corbell on Creative Live (if you don't have an account yet, GET ONE! Creative Live is one of the best resources you can have) because the depth of knowledge is fantastic and you come back to it any time you like.