Here is a quick yarn weight guide that you can print out to use when browsing crochet and knitting patterns or when buying yarn. Some yarns are labeled with the image, but sometimes they only give the verbal description of the yarn.
It is important to make sure that you use the correct weight of yarn for the project you are intending to make to ensure that the finished piece is the proper size, fit, and drape. It is also great to know what the recommended crochet hook or knitting needle size to work with if your pattern doesn’t specify, or if you are making your own pattern.
Of course, the bulkier the yarn, the quicker it will work up and the larger hook/needle you need. Thinner yarn is great to use to make finer items like doilies and delicate lace, but is sometimes more difficult to handle.
Try out some projects with a variety of yarn weights to get a good feel for them.