Casting on is a simple term. It means to put stitches on a knitting needle to begin a project. Simple, right? Not always.
To begin, there are MANY types of cast ons, so many that I am only loading one video on this page. I will use links notated by BOLDED text for other videos. The first cast on I ever learned was the E-Wrap or Reverse Loop cast on.
As you can see this is an extremely easy cast on. It is a twisted loop on the needle. It usually resembles a cursive "e" and that is why it is called the E-Wrap (Reverse Loop). You can create it to lean to the right or to the left by switching the direction that the yarn wraps around the needle. This was the first cast on that I learned. I was young and I was learning knitting from a small book (there was no internet back then). The pictures showed the cast ons. I remember seeing another cast on with hands and many arrows showing the direction that the yarn should be wrapped. I could not figure it out from the pictures, so I used the Reverse Loop. There are a few problems with it. It can be loose and can be uneven for an inexperienced knitter. It is not a stretchy cast on because there is only a twisted loop of yarn with no excess for 'give'. However, it was the only cast on I used for 20+ years. So I know you can knit almost anything using just this cast on.
Many knitters are taught the Knitted Cast On as their first cast on. This one is great for beginners because it is the basics of a knit stitch. Your hands 'learn' how the knit stitch feels and you progress forward from there. A close cousin to this is the Cable Cast On, which uses the exact same technique as the Knitted Cast On, but you start by putting the needle between the stitches instead of into a stitch. The Cable Cast On isn't as stretchy as the Knitted Cast On, but it is a very good structural cast on. One advantage of a Knitted Cast On, Cable Cast On or Reverse Loop is that these can all be done in the middle of your work. You can add stitches at the beginning or ending of a row for shaping.
I didn't learn the Knitted Cast On or the Cable Cast On until rather late in the game. The next cast on I learned was that confusing cast on from the pictures in the book. It is called the Long Tail Cast On.
This is an easy cast on once you get the hang of it. It is quick, flexible and will work for many different projects. It gives more stretch than the Reverse Loop because there is more yarn involved and the yarn can 'borrow' from the other cast on stitches. It isn't known as a super stretchy cast on, but it works for many projects. Between these two cast ons, I have been able to do hundreds of knitting projects. Again, it is versatile and universal.
There are many variations to the Long Tail Cast On. There is the German Twisted Cast On, Channel Island Cast On, Italian Cast On and many more. You can even do the Long Tail Cast On with two hands. All of these methods do something slightly different and have different amounts of stretch.
Then you get to the Provisional Cast On.
There is more than one way to cast on provisionally. You can use waste yarn, or an extra flexible needle. There are also decorative cast ons. In all honesty, there are books based solely on cast ons. I could go into every cast on in existence, but this is a blog, not a book.
How do you know which cast on to use? This is one of the reasons that casting on isn't quite as simple as you think. You may have a reason for doing one cast on over another. You may need more stretch or more stability. You have to decide what is required in your cast on and make the decision accordingly. Patterns will tell you if you need a specific type of cast on (provisional, ribbed). If no cast on is mentioned, or the pattern says to use your favorite cast on, then you get to make the decision.
When I am starting a project, I decide what type of structure my cast on requires and then I research cast ons to fit that structure. For toe up socks, I usually use a the Turkish Cast On. For cuff down socks, I will use a stretchier cast on called Jeny's Super Stretchy Cast On. For most projects I use the long tail cast on (see above link).
Trust me, cast ons are a rabbit hole that you can fall down into if you really want to do the research. This is NOT a complete list of cast ons. You can cast on many ways for many different items. There is not going to be one perfect cast on for everything, but you can use one cast on for (almost) everything if you really want. I used the e-wrap for over 20 years before I looked for any other type of cast on.