What does aspect ratio mean?

The simplest explanation of aspect ratio is that it has to do with framing. Take a look at the examples below.

The above photo with the clowns was shot using a widescreen lens, which has a 16:9 (sixteen by nine) aspect ratio. Simply put, 16 units long for every 9 units high. 

The same photo above has been cropped to a 3:2 aspect ratio, which is very common for most DSLR cameras today. As you can see to the left and right of the photo, some of the original picture has been cut off.

Here we have yet another example of how different the photo can look using another aspect ratio of 4:3. This time, even more of the photo has been cropped on the left and right sides. 

Which aspect ratio is best?

When discussing which aspect ratio is "the best," there really isn't an answer. It really comes down to style and preference. It also depends how you would like your framed print to look when it's done. This brings up another interesting point, which is talking about what you will lose when you mat the photo and put it in a frame. 

The above photo shows the 4:3 aspect ratio, and the light blue square represents the cropping that will occur when the photo is place under a mat. About 1/4 inch all around is needed, so you will lose another half inch of the photo after it has been mounted. 

What can be done so I don't loose any of my photo to cropping?

There are some solutions. Firstly, you could have the photo printed on a metal plate (aluminum) so that you don't have to crop with a mat, but you will still lose a little because the aluminum plates come in standard sizes. 

The other option would be that you could print the original photo without any cropping, and just have your mat and frame made custom. This, of course, is a more expensive option, but it is a solution to the problem. 

One last note on aspect ratios

Remember that some photos have a horizontal orientation and others are vertical. The same numbers are used. So a 3:2 photo could also be a 2:3 photo.