After you know slip stitches, half-double, double, and treble crochet, you'll be ready to learn things like the popcorn stitch (which creates the impression of a ball in the fabric) and front and back post double crochet (which allows the crochet to look almost ribbed, like knitting). A stitch guide like Vogue's Stitchionary Number Four will help.
If you are a beginner, then the plain/base crochet stitch will be the one you need. If you are a learner, then are couple of interesting stitches like the chain stitch & cluster stitch. With these two stitches you can make items in yarn and wool ranging from baby's clothes to a granny's scarf.
When learning to crochet, shell, lace, open weave, textured, and rippled are the most unique and sometimes difficult stitches to master. There are also patterns called Irish crochet, diagonal crochet (a method of crochet done corner to corner)and punched fabric. Shell crochet is the most beautiful of all crochet techniques.
Try the cable stitch, where you front post, stitch two to five times on one row, then back post, stitch the same number of stitches on the next row. You can use single, double or triple crochet to do the cable stitch. Use it to make just about anything with a braided effect like purses or sweaters.