I know there are those of you out there searching for the difference between these two stitches, and I'm here to tell you I totally understand your pain. It's taken me a long time to figure out the difference, and it turns out that's because there really isn't much of a difference in the first place - they're basically upside down versions of each other!
If you're working a pattern that calls for stem or outline stitch, but you only know how to do one, don't worry. You can simply flip your hoop upside down as needed to work the pattern with the stitch you're most comfortable with, or you can learn the difference along with me right here!
In the video below (also available to watch on Instagram), I work an outline stitch on the top curve and a stem stitch on the bottom curve to demonstrate the difference between these two stitches:
From this video, you can see that what I'm essentially doing is keeping my loop to the outside edge of my curves - if I flipped my hoop around and stitched upside down, suddenly the outline stitch would become stem, and the stem would become outline. That's what I meant when I said they were basically upside down versions of each other!
But what happens if you use outline stitch when you maybe should have used stem stitch? I do my best to explain what that might look like with the image below:
If you're still feeling confused, I'm here to give you permission to seriously not even worry about it. If you're working a pattern with outline or stem, it's most likely safe to assume that you're going to be stitching with your loops held to the outside of your curves. That is basically what I've done for the past five years or so, and I've been just fine! Don't worry if you're using the right words or whatever, there aren't really any hard and fast rules in the world of modern hand embroidery.
As always, I really hope this has been helpful - whether you're using this tutorial to help you with another artist's patterns or kits, or to create your own unique pieces! If you're a beginner yourself, I have a ton of embroidery kits to choose from that come at a few different price points and are all geared toward being easy to follow even if this is your very first project. Happy stitching!