The vowel chart is an extremely useful tool in learning dialects (including the neutral American accent) because it's a map that shows how parts of the mouth move to make the various vowel sounds. Understanding with the vowel chart makes it possible to learn new sounds physically, as opposed to relying on the ear, which may not be able to hear unfamiliar sounds. Your ability to hear the new sounds will improve as you learn to form them physically.
The vowel chart (Figure A below) fits into the middle of the mouth as shown in the simple cross-section of the head (Figure B).

The role of the tongue: Note that the left side of the chart points toward the lips, and the right side points toward the throat. The upside down, backward "e" symbol (schwa) in the middle of the chart would be located in the middle of your mouth.

Each point on the chart is a position in the mouth where the tongue moves to make a different vowel sound.

Which part of the tongue? It's simplest to consider the front of the tongue (not to be confused with the tip, which is the part you see when you stick your tongue out.) Behind the tip is the blade, then the front, middle and back, as shown in Figure C.

Put another way, the front of your tongue lies directly beneath your hard palate (the domed part of your mouth behind your gum ridge) when your tongue is at rest.

Now try some vowel sounds: Look in a mirror and say the sound "eeeee" as in the word "see." Next say "aaaah" as in "father." Alternate saying these two sounds a few times: "eeeee, aaaah, eeeeh, aaaah."

Do you see your tongue moving in the mirror? Notice that the front of your tongue moves up and forward to just behind the gum ridge for "eeeeh," then down and back for "aaaah." You may also notice that the tip of the tongue is behind the upper teeth when you say "eeeeh." Remember, however, that we're concerned right now with the front of the tongue, which, though hard to see in your mirror, is up and forward, near the gum ridge for "eeeeh."

If you look at the vowel chart, you'll see that the phonetic symbol for "eeee," a lower case /i/ is located in the upper left hand corner of the chart, or as far up and as far forward as the front of the tongue moves to make a vowel.

Now say "aaaah" as in "father" and notice that your tongue moves down and back. On the vowel chart, this sound is shown in the lower right hand corner with a lower case /a/ symbol, just above the end of the word the word "father."

In the same way, each symbol and key word on the chart represent a position in the mouth to which the tongue moves to form a vowel. If you don't move it anywhere, and just leave your tongue relaxed in the middle of the mouth and let out some voiced air, you get the sound "uh," shown in the middle of the vowel chart with the "schwa"symbol, an upside down, backward "e." This is a very important sound for the American accent, as it's the most common vowel sound heard in American English. We often pronounce spelled vowels with this sound, especially when they're in unstressed syllables. The underlined vowels in the following words should all be pronounced as "uh": allow, deny, possible, compose, upon.

The lips and jaw help, too. In addition to how your tongue moves, it's also important to notice how the lips and jaw shape vowel sounds. If you consider the vowels along the front of the vowel chart (moving down from "see" to "cat") you'll note that your jaw is slightly open for /i/ (eeeeh) like in "see" and opens more as you move down the chart to the vowel sound in "cat."

Along the back of the chart there is lip-rounding: When you say the vowel in "blue" at the top of the back, your lips round very fully. As you move down the back of the chart, they round less and less until you don't round them at all for the "aaaah" sound in "father".

Explore the vowel chart: Try making a few of the sounds by reading the key words. Check yourself to see if your tongue and lips where they should be according to what you've now learned about the chart. Check yourself by looking in a mirror.

We've already discussed the vowel sounds "eeeeh" and "aaaah"; try making the vowel sound "ooooh" as in "blue." Your lips should round very fully, and the back of your tongue should raise up near the roof of your mouth. Your tongue tip should be relaxed behind your lower front teeth. In many Southern American accents, the tongue tip raises for this sound, giving it a sound like "ehooooh". But for the more neutral sound the tongue tip should stay down, with only the back of the tongue working.

Knowing the vowel chart makes it possible to learn new sounds physically, as opposed to relying on your ear, which can't always hear sounds that aren't used in your own language. As you get better at forming the new sounds you'll find that you'll be able to hear them better, making it easy to learn new dialects.

What's the difference between a vowel and a consonant?

In general, with vowels, the working part of the tongue comes close to, but doesn't touch any other part of the mouth. With consonants, two parts of the mouth touch: for the /p/ sound, the lips come together then explode apart; for the /g/ sound the back of the tongue touches the soft palate, then explodes away.

Additionally, there are four semi-vowels, in which the articulators (the working parts of the mouth) come close together but don't touch: /w/, /y/, /r/ (the American version) and /h/.