8 ply wool (or yarn) is probably the most commonly used ply for knitting, and is available in a wide variety of colors and textures.
This ply is also commonly refered to as double knit or medium weight depending on which country you live in, and can be used for a virtually endless variety of knitting projects including:
One way of measuring the weight of your yarn is to count the number of wraps per inch.
You can use any ruler to do this (as I have in the photo), 8 ply yarns will wrap around your ruler at the rate of 13 wraps per inch.
The normal tension for knitting in 8 ply is around 22 stitches and 30 rows over a 10cm square on 4.0mm knitting needles.
If you are a first time knitter or still consider yourself a beginner, 8 ply is probably the best choice for your knitting projects.
Because it is the most commonly used ply, patterns are readily available and many of them are designed to be knitted by beginners. If you have a look at my beginner knitting patterns page you will find several patterns for childrens sweaters and scarves that are knitted in 8 ply.
If you are going to purchase a pattern (or a pattern book) just check the skill rating on the patterns to select one appropriate to your skill level before you buy them. If you are going to make a garment with a ribbed edge, you will probably need a pair of 4.0mm and 3.25mm needles to complete your garment.
Having said that, you really should knit a tension square to determine the correct needle size for your knitting tension.