Just to add a note to this:
If you want to use a computer to record content to be used on YouTube or Twitch, and you don't want your audience to see an image that is stretched, squeezed, or bordered by black boxes, or cropped, then the display you record on needs to be the same aspect ratio that YouTube and Twitch presents to users, which is 16:9. The common variants are 1440x900, 1920x1080 and 1280x720. If your screen's native resolution has a different aspect ratio than this, then the image will end up being modified to fit this ratio, and either you (while recording) or your audience (while watching) will see this modification, which can be cropping, bordering, stretching or squeezing, depending on what the difference is between your display's aspect ratio and the 16:9 standard.