A diphthong (pronounced "DIF-thong" or "DIP-thong") is a vowel that the speaker has to move their mouth into two different positions to make. It is a vowel where two different vowel qualities can be heard. Examples are: waist, die, noise, road, house, fierce, bear, sure. Each of these is a different vowel sound.
A monophthong is a simple vowel sound that a person does not have to move his or her mouth to make, like the "oo" sound in "book." In a diphthong, the person combines two different monophthongs, as with the "oi" sound in the word "oil". The speaker starts with the mouth in the position to make an "o" sound, then quickly moves the mouth to make a hard "e" sound. Another example is the "ou" sound in the word "house". The mouth starts out making a sound like the soft "a" sound in "flat", then moves to make the a hard "oo" sound like the one in "caboose".
Just like with every other part of language, the exact way to pronounce a diphthong is a little different for different accents.
The word diphthong is derived from the old Greek language. Here, di means two or double, while the part -phthong means sound or tone, from the basic word phthalein, which means speak, creating sound by the voice.