If you have finished off the old color, place a slip knot on your hook, insert hook where you want to join the new yarn, yarn over and pull through both the stitch and the loop on hook (making a slip stitch). Chain up however many you need for the stitch you will be making next and continue working with the new yarn.
If you have not finished off, but are just continuing from your present point, work your last stitch before the change in color or skein to the last step, then just lay the new yarn over the hook and complete the stitch with the new yarn.
Another name for a hank of yarn would be a skein of yarn.
skein
Yes, you can join leftover yarns together to make skeins of yarn. There are no size limits to the skein that you create, make it the size that you want. There are many ways to join yarn--a simple knot will do the trick. Personally, I like the Weavers Knot (see attached video). I also added a video which shows braiding the ends together. Friends I know use the "spit" method, if they are using natural fibers. Also, you can weave in the ends. But, if you have a lot of ends, that can be a project in itself.
Yarn comes in different "gatherings"--ways that it can be sold. Your question mentioned 2 out of the three: ball of yarn and skein of yarn. The third would be a Hank of yarn. A hank is a skein of yarn (which is yarn in a large oblong shape, and tied in a few places around the skein to maintain a bit of control over the yarn staying in that loose shape), which has been twisted around itself to make for a smaller and more controllable yarn, which is easier to pack, and store. A hank also is a bit more manageable than a skein of yarn. A ball of yarn would be wound from a skein or a hank.
A coil of yarn may be called a hank or a skein. Yarn is typically sold in a ball or a skein. A hank is different in that it is a big loop of yarn that is twisted up for easy shipping.
I believe that this is due mainly to the fact that the machines that wind the yarn can only wind it in a skein. Also, since the yarn is wound loosely in a skein, it appears to have more yarn than if it were in a ball, which is better business for the yarn company. Because of it's shape, a skein is easier to store than a ball.
While there are general standards, there are no absolute standards for the amount of yarn in either a skein or a hank. The amount depends on the weight of the yarn and which fibres are including in its spinning.
Yes, a small amount of wool yarn can be called a skein, especially if the fibre is rare and expensive, such as baby yak wool yarn.
A flock or a skein (as in a skein of yarn) (pronounced skayn)
The way I learned to join another yarn to my crochet project is to add the new yarn to the stitch you are doing, when there are two (2) loops of the stitch left on your hook.For example, I'm going to add new yarn to a double crochet project. So I would do my dc (double crochet) stitch by beginning with:yarn overinsert the hook into the next stitchyarn over hook againpull hook through stitchyarn over hook againnotice that you now have the last two loops on your hook, this is where you bring in your new yarn,grab new yarn with your hookpull through the last two loops remaining on your hookyou have now completed the dc stitchcontinue your project as written.
The length of yarn in a skein varies greatly with brand, variety, etc. You can determine the length of the yarn by looking on the front of the label, where it is listed along with weight, recommended needle/hook size and dye lot
I believe the spelling is skein. It is a package of yarn looped to make it easy to pull a yarn thread from the center of the skein and either use it or wrap it into a ball. This is how it is typically sold. However, there are some stores that sell yarn in balls.