OK, a dipthong is a vowel sound, but it's actually made up of two - or more - vowels (although strictly speaking if it's 3 its a tripthong).
You are used to dipthongs. They are often written using multiple letters. You may or may not have ever thought much about this but there are plenty of examples.
The word "eye" is a good example. And indeed, the word "I". Two ways to write a well-known dipthong. If you say it very slowly you'll find that it actually begins, briefly with the sound "ah". It ends with the sound "ee". In the middle, when the the former slips into the latter you can definitely hear that yod sound ("y").
And curiously the letter y, pronounced why, is a tripthong. Say it very slowly.
Do this and be aware of the shape of your mouth.
This is why w can actually be considered a vowel, and is in some languages.
Later I'm going to throw in two more letters that you think of as consonants, that can be vowels, but let's look at dipthongs a bit more first.
Depending on your accent (there it is again) you may pronounce some of your vowels as dipthongs, or pure vowels. It isn't that accent A has more dipthongs than accent B, it's just a pick and choose thing. Scottish, for example, is often said to have pure vowels, but it also has plenty of dipthongs, just said so quickly that only an obsessive language geek can hear them.
In other accents it's really easy to hear them.
There's a sterotype, in fact, that certain accents (such as Cockney) is full of dipthongs, but perversely it also changes some "normal" dipthongs into pure vowels, which is why a Londoner can live in an ass. Many a confused American has been asked to "come rand my ass lighter". I daresay few actually arrived.
But it's this switch from pure vowel to dipthong that has caused Americans to talk of Hairy Potter, and taking their car across to the island on the fairy.
(OK, that's a fairy on a car, but it was the closest I could find).
So what is a pure vowel? It's when no matter how much you slow it down, it stays the same from beginning to end. Some of the purest vowels in the world can be found in South Africa, or perhaps I should say Suth Efrica. There are a few dipthongs there, but you'd hardly notice them. This is possibly why it's one of the nicest accents to listen to, and I could listen to Trevor Noah all night (I wonder if he's done any audio books).
This brings me to an interesting fact.
People really do feel strongly about accents. They have preferences and find some soothing, some grating. And accents revolve around vowels and dipthongs. On the whole, the accents with a lot of dipthongs are the ones we see as less "educated". Why? Because prestige accents in most parts of the English speaking world tend to have more pure vowels.
(This won't embed, but do watch it: https://www.youtube.com/watch?v=0OFXL0jIMR4)
And Cockney, with its many dipthongs is the accent of the common people.
What you won't find in Cockney, or in the poshest London accent, is a rhotic vowel. Not even among pirates.
There isn't full agreement among linguists about this (just in case you thought this was an area of study which was cut and dried, thankfully linguists argue, just like all experts) as to whether such things as rhotic vowels exist. There is another term - the R-Coloured Vowel, which is another way of looking at it.
An example would be my poor husband's name - Martin. Most people here in Canada call him Mrtn. No traditional vowels at all. The R becomes the vowel. How can that be? Well, if you slow it down, you'd be hard-pressed to pick out which vowel came before it. I think it's a schwa (an unstressed vowel, i.e. "ugh"), if at all, but it's also possible that R really can be a vowel.
What it certainly CAN do, for non-Rhotic speakers (most of England, and parts of the Southern US, as well as Boston) is affect the vowel before it. Again, with Martin. The way we (non-rhotic speakers) say it, it sounds like Mahtin. The R has lengthened the A. Without it, it would be Matin. But we don't pronounce the R itself.
How did the R become - well - silent?
Because German is non-rhotic. It has been for a long time, nobody really knows how long, but it was certainly non-rhotic by the time Hanoverian kings arrived in London in the early 18th century. It's widely believed than their pronunciation was at first copied as an affectation by the elite at court, spreading quickly as a trend through the aristocracy, down through the gentry, and reaching the lowest social groups well before Dickens' time. It also spread out to other cities, and beyond, albeit slowly. 50 years ago there were still plenty of rhotic speakers in the countryside in Southern England, but they are rare today and it's a generational thing in some places.
But Londoners are very creative people and have managed to turn L into a vowel.
I've spent most of my life trying to deliberately refine my accent, but unless I'm really, really careful, I still do this. Milk? Mee-awk. Walk? Wawk. Etc. Still, my Canadian kids say WAHK for walk, so maybe it's not just Londoners.
And there, of course, H becomes a written vowel, even GH, sometimes, or at least part of one. Can you think of any more?