As for cassette ratios, the small numbers and large numbers are important. The smaller the small number, will give you a higher maximum gear. The larger the larger number will give you a smaller minimum gear. So why not go for the biggest spread like an 11-32? Well, that causes bigger gaps between gears. When you are riding, making small changes to your gearing is a big help. That is why gearing like 12-23 is popular.
On the chain rings like 53-39 a similar thing is happening. The bigger the big number, the larger you maximum gear. The smaller the small number, the smaller your minimum gear. A triple chain ring gives you a bigger range with a slight degradation of shifting "crispness". Also triple chain rings are viewed by many road riders as not stylish. Mountain bikes on the other hand, seem to always be triples.