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
Depends on the yarn. There is yarn that is 100% cotton, but cotton/rayon mixes and other mixes are common.
Another name for a hank of yarn would be a skein of yarn.
skein
To skein yarn without a niddy noddy, you can use a chair or your arm to wrap the yarn around in a loop, creating a skein.
There isn't a way to figure out how many skeins of worsted yarn would equal 18 oz. You would need to look at the yarn, and see how many ounces are in that particular skein and go from there.The word "weight" in yarn refers to the size or thicknessof the strand of yarn, not an actual ounce, pound, gram or kilogram. Then the size or thickness would be measured by how many stitches are in 4 inches. The Yarn Council of America has a guideline for "weight" measurement for yarns.The reason there are variables in the actual skein ounces, grams weight, is that different fibers of the same length would weigh differently--some are lighter or heavier than others.
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.
The yarn that is attached to the skein is often referred to as the "tail" or "end" of the skein. This tail is used to pull off the yarn for knitting or crocheting without unraveling the entire skein. In some cases, it may also be called the "working yarn" once it is being used in a project.
Since the regular Red Heart skein is four ounces, the pound package is four skeins for your conversion chart.
A skein of wool can come in different weights. There is not a specific amount of wool needed in a skein.
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.
A small amount of wool yarn is typically referred to as a hank rather than a skein. A hank is a looped bundle of yarn that needs to be wound into a ball before use. A skein is a different form of yarn packaging and usually contains a larger quantity of yarn.
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.