I was wrong when I said that stem and outline stitch are the most common stitches. The straight stitch is even more basic. It can be stitched in any direction and any length and it forms the basis for many other stitches. This stitch is so easy that I almost don't need to show pictures, but I will anyway.
Bring your threaded needle to the front.
Take the needle to the back at the end of the stitch and come up at the beginning of the next one.
Pull the thread through making sure you don't twist the thread. You want it to lay flat and, oops, don't look too closely at my example because I didn't do this. At your last stitch keep your thread at the back and weave it under your previous stitches. Easy peasy.
Used alone, this stitch is great for detailing—like in these individual pine needles
For animal fur (and as the basis for those yellow cross stitches)
Combined with outline stitch for shading in leaves and flowers
Here's another flower, and you’ll notice that the sepal is also done in straight stitch but with densely packed together threads. This is called satin stitch.
When I work straight stitch I often use more strands of floss than I would for a stem or outline stitch. Those stitches become thick because of the braiding and overlapping of the threads and you don't get that with straight stitch. I've found that four strands is about right for most things.