What happens when you fry food?

It's called the Maillard reaction, after the French chemist Louis Camille Maillard.

This is a chemical reaction between carbohydrate groups (that's sugars, largely) and proteins or amino-acids (those are the building blocks of proteins).

It takes place at around 148 degrees Celsius, and when these sugar groups lock on to the amino-acid groups they form these nice, brown caramelised substances which taste great - that's the nice aroma you get from cooking.

As it happens at 148 degrees - that's 48 degrees hotter than water boils at - that's why you get a very different consistency and taste and texture to fried food compared to boiled.