Worsted weight is a medium-thickness yarn (weight category 4) and the most popular yarn for crochet and knitting. It typically knits to about 4–5 stitches per inch and suits blankets, sweaters, hats, and most everyday projects.
Worsted sits in the middle of the yarn-weight scale — thicker than DK, thinner than bulky. Because it's the default for so many patterns and works up at a satisfying pace, it's usually the recommended starting weight for new makers.