Back in the days of film photography, you had to think very carefully about the aspect ratio of your image before you shot it, and different aspect ratios held enthusiastic fans that specialized in their usage. The advent of digital and the standardization of the sensor into just two aspect ratios has seen that focus die away a bit, but they remain highly important in how your images are rendered, and this excellent video tutorial discusses their usage.
Coming to you from Alister Benn with Expressive Photography, this insightful video tutorial discusses the use of aspect ratios in landscape photography. Modern digital sensors are almost always in the 3:2 or 4:3 format, but of course, you can easily crop to whatever you want in post. That being said, if you think through this in the field, you can avoid shooting overly wide and losing too much resolution through cropping in post, and you can tailor the composition to the desired aspect ratio, making for stronger overall images. Check out the video above for the full rundown from Benn.
And if you really want to dive into landscape photography, check out "Photographing The World 1: Landscape Photography and Post-Processing with Elia Locardi."