Probably the most important part!

Yarn varies in feel, wool (and silk, and angora, and others!) content, bulkiness, colour and shading, and price almost infinitely. 

The yarn you choose is obviously fundamental to the final feel, fit (how it sits and hangs), warmth, and strength of your projects.  For example, if it's a teddy or toy, the yarn will affect how well it stuffs and keeps it shape. Or if it's a cover for something, the yarn is important to the wool's "memory" and hardiness. 

Suffice to say there's definitely a yarn for every project, but not every project will suit every yarn. Again I'll do my best to find the best suited and reasonably priced. If you've got your own yarn you'd like me to use, I'll be honest about how I think it will turn out. Am happy to go shopping with you too if you'd like.