When buying yarn it is important to know what is being bought.
The labels on the yarn tell you a lot about what you are looking to buy. That is of course if you can read the label. Here is a quick way how to guide to show you the basics on Yarn Label Reading
Step 1: Weight and Length
You can't really tell from just holding it how much yarn there is in a skein. Luckily on the label it tells you how much yarn there is. Just look by the net weight, often times the yards and meters are also listed.
This will help you estimate how many skeins you will need to buy for your project.
Step 2: Fiber Content
On the label there will also be a percentage, something like 100% acrylic or 50% wool 50% cotton. This tells you what the yarn is made from.
So if you want to make a wool sweater look for yarn labels with 100% wool on them
Step 3: Needle Size
On every label there will be two squares with either two lines or one line in it. These depict kitting needles or croquet hooks. Around the square will be the size of needle recommended for this yarn. It will be listed in both mm and US.
Step 4: Laundry Directions
Finally there will be laundry directions located on the label stating weather it is machine washable, machine dryable, and ironable,
While you may come across many symbols here is a chart of what you will probably find. Make sure to double check before you decide to machine wash or dry anything.