In my opinion, shading is one of the most important techniques when it comes to drawing and sketching. It gives your artwork perspective and "life". When using shading, artists use different tones. These tones are based on the pressure of your pencil. The lighter you press with your pencil, the lighter the tone is. The harder you press your pencil, the darker the tone is. There are five main types of shading: smooth shading, hatching, cross-hatching, stippling, and scribbling.
Smooth shading is one of the more difficult types of shading. It requires a very smooth (obviously) and almost perfect looking shade (I am definitely a perfectionist, so I use this type a lot). The trick to the smooth shading is to hold your pencil differently. Here is how you should hold it:
Hatching is where you shade diagonally and you don't want the lines to blend together. You also might want to use the hand position I requested earlier, also.
Cross-Hatching is the same thing as hatching except you want to cross the hatching.
Stippling is more of a dotted form of shading. To make a lighter shade, you want to use less pressure on your pencil and make your dots farther apart. To make a lighter shade, you want to use more pressure and make your dots closer together. You probably would prefer using a normal hand position when holding a pencil for this type of shading.
Scribbling is well, scribbling. For lighter shading, use less pressure and for darker shading use more pressure.