How do you like your photos? Wide, or tall? While there is no right answer in art, the conventions would usually dictate that if your subject is of a vertical orientation, then a tall photo would be better suited.
When shooting a photo of a person, we would usually choose portrait orientation.
When shooting a landscape scene, we would normally .... I'm not going to finish the sentence. You know.
Of course there are always exceptions. For example when we want to include some of the scenery in the picture with our person subject, it might be better to use landscape orientation. Or what about using portrait orientation for a scene where you want to show off a lot of the dramatic sky.
As humans, our eyes are horizontally placed, probably to align with the horizon. As such our field of view is wider than it is tall. Check for yourself. Keeping your eyes still, you can see more left-and-right than top-and-bottom. Landscape orientation is natural to us, and that is probably why camera manufacturers default to landscape.
To get a photo in the portrait orientation, I don't have to tell you that you need to rotate your camera 90 degrees. That's too obvious. But 90 degrees in which direction? Watch the pro photographers. Almost every one will rotate their right hand up towards the top of their head. This is a very important technique. With the right hand up, it leaves the left hand to continue to provide support underneath the lens and take the weight of the camera. It also means the left arm doesn't need to move and remains locked in against the chest for stability.
If you see someone shooting in portrait orientation where they move their right hand down to their chin, watch where the left hand goes. It just flaps about, looks silly and aids in the camera wobbling about. It's okay to point and laugh at people who shoot like this regularly.
How tall and how wide? The aspect ratio describes this. Remember, the olden days TVs were quite square, and then suddenly widescreen TVs were all the rage. The old TVs - for technical reasons not worth discussing here - had an aspect ratio of four by three. Or 4:3. Regardless of the physical inch size of the screen, the width was four units, and the height three units. Our widescreen TVs are 16:9. Or to put it in fractions, the screen is 1.777777 times wider than it is tall.
Wikipedia has a graphical description of the different aspect ratios: https://en.wikipedia.org/wiki/Aspect_ratio_(image)
The sensors in our cameras are rectangular and have an aspect ratio. I'm sure there are exceptions, but generally, DSLR cameras have sensors with 3:2 ratio, and most "smaller" cameras have a ratio of 4:3. Remember this has nothing to do with actual millimetres or megapixels - this is the shape of the sensor.
The aspect ratio is important to keep in mind when composing your image. What shape your sensor is relative to how you intend the image to be displayed is relevant.
If you get your photos printed, the standard size is 6x4 inches. The ratio is? (Divide both numbers by the common denominator - in this case 2.) That's right, the ratio is 3:2, which is great if you have an SLR type camera with a 3:2 sensor (or film!).
But what if you have a point-and-shoot camera, or want to print photos from your phone's camera? These images are likely to be of 4:3 ratio, so will not fit nicely onto a 6x3 inch piece of paper - the shapes are different. You are going to have to crop.
Cropping is done in software, and it is the process of chopping off some of your image.
A lot of the time you will probably crop an image to get a tighter composition, or to remove some unwanted element to the side of the frame. Often without changing the aspect ratio.
It is fine to change the aspect ratio to anything that makes artistic sense for your particular subject. But keep in mind how the image will be displayed. If you are printing, and the image is to be framed, what aspect ratio is the frame? If you want to use the image as a background on your computer screen, then you will benefit from cropping the image to a 16:9 aspect ratio to match that of the screen. If you will be posting the image to Instagram, then a 1:1 (square) aspect ratio might work best.
In the series of images above, I've cropped the same picture to the various aspect ratios mentioned. 16:9. 3:2. 4:3 landscape. 4:3 portrait. 1:1. The 3:2 image is obviously the original size out of my camera. Notice when we crop, and thus lose a bit of the image, the subject can seem larger. (Click on the images to see what the bugs are doing on the flower.)
Here's a task for you: Find out the aspect ratio of your camera's sensor. Then compare that to that of your mobile phone's camera. (Maybe take the same photo with both cameras.) Compare these photos to the aspect ratio of your computer screen. How will you crop each photo so that it fits on the screen perfectly, while maintaining your artistic intent?