When the term 24 STS is used with respect to crocheting, it means that there should be 24 single crochet stitches across one row. When ST is used by itself, it refers to stitches, and STS typically refers to slip stitches.
There are several other crocheting terms that are wise to know before beginning a pattern. The term "beg" refers to begin. BL or BLS, similarly to stitching, refers to one or more back loops. DEC refers to a decrease in stitching, whereas DTR refers to a double-triple crochet. SP and SPS refer to spaces, and CH or CHS refers to chains within the crochet.