There are numerous stitch types depending the embroidery decoration that is being done.
The more common options are satin, fill, running (single) stitch and balboa.
The satin stitch is used more frequently for lettering.
The fill stitch is used for large embroidery areas such as circles and squares. It is also used in lettering that has thick columns or being sewn on thin material to prevent pulling and puckering of the garment.
Balboa (trapunto) stitches are primarily used when embroidering on items with a knap or thickness to them such as fleece or knit caps. The balboa stitch is sewn down to hold the material knap down or hold the columns of the knit cap together and is done in the same color as the garment. This helps to keep the design from sinking into the material or columns of the item.