straight stitch is the simplest embroidery stitch, and is the basis for many other stitches. it is, simply, a single stitch.
a row of straight stitches in a line is called running stitch, which looks like a dashed line. usually, you want to make each stitch the same length, and each space between stitches the same length. for running stitch, i like my stitches to be about 1/4 inch (about as long as a grain of rice), and each space about 1/8" (about half a grain of rice).
if you scatter straight stitches randomly, they are called seed stitches. you can make them as long or short as you like, as thick or thin, as loose or dense; you can use multiple colors or all one color... the variations are up to you.