All our worsted and aran (heavy worsted) yarns. This is the middle of everything yarn weight. Generally about 18-20 sts on a US 7-8 needle.
Yarns come in all kinds of thicknesses, from lace weight to jumbo. Generally the thinner the yarn, the smaller the needle or hook size you use. Conversely, the thicker the yarn, the bigger the tool size you use. But, of course, you can often use a different tool size than recommended - not only to obtain gauge, but to create a different kind of fabric - drapier or firmer, depending on your desire.
Find a chart of the standard yarn weights here.