American English sounds can be broken down into two major groups: the consonant sounds and the vowel sounds. Within these two groups, sounds are divided up into smaller groups. based on the features they have in common.
To describe what voicing is, we really need to understand what makes our voice. The vocal cords, which are located in your throat, are what are responsible for creating each person's unique voice. They look like two little rubber bands and can move in three different ways. Just for a minute hold up your index and middle fingers on one of your hands in the shape of a "v." This is what your vocal cords look like, and they sit horizontally in your throat.
Now, imagine your index and middle finger coming together and then separating over and over very quickly. This is what happens when you are speaking. Your vocal cords move together and apart so quickly, anywhere from 150 to 200 times a second, that they create a vibration in your throat. To test this out, let's see what your throat feels like when you speak. Place one hand around the front of your neck like this, and say "zzzzzzz", which is the American English pronunciation for the "z" sound...."zzzzzzzz". Two things happen: first you can hear your voice, and second you can feel a vibration when you say that "zzzzzz" sound.. This type of sound is called a "voiced" sound, because your vocal cords are moving or vibrating.
Even though it seems like we always using our voice when we speak, we really aren't. Sometimes we are just push air out of our mouths in different ways, depending on how we position our lips and tongue. When this happens, our vocal cords are apart and in the shape of the "v" that we talked about. Make the shape of the "v" again and look at the space that you see in between your fingers. If your vocal cords aren't moving, you can't make sound. All you can do is take air into your lungs and push air back out through your lungs and out through your mouth/nose. Any time you breath, the air you take in has to go through your vocal cords. To breathe out, you push air past your vocal cords again. Your vocal cords, then, are important not only for speaking, but for breathing as well. Let's see what your throat feels like when you just push air out. For this example, we'll use the letter "s", which say "sssss" in American English. So again, place one hand around your neck, and say "sssss." Now two things happen here. You hear air being pushed out of your mouth, and second you feel nothing in your throat. This is because your vocal cords are apart or open, which means that they are not moving. This type of sound is called an "unvoiced" sound. One important factor to remember is that consonants can be voiced and unvoiced, but vowels are always voiced.
So far, we have talked about two positions that the vocal cords can move in: one where they are moving together and apart, and the other where they are open. There is a third way that they can move and this is together. If your vocal cords are together, there is no space between them. This is what we say when the vocal cords are closed. When this happens, you are holding your breath. Every time you go under water, you put your vocal cords together and hold your breath to stop breathing. Some sounds in English start by holding your breath. So you will have to know each of the different ways that the vocal cords can position themselves when you learn how to pronounce American English sound.