The DNA contains the order for the genetic coding for the organism, it literally translates the order of proteins and how they are arranged to make the organism the unique creation it is. It is the "why-all" and "be-all" about what the organism is, what it looks like, how it typically behaves, and such.
When you change the order of the base pairs in an organism, that is called a mutation. There are three types of mutations: a deletion, an addition, and a substitution. In a deletion, one of the three bases in a codon is pulled out, or deleted. In an addition, an additional base is inserted into one of the codons. In a substitution, one base is substituted for another.
In these genetic manipulations, one of three things can happen: something positive or beneficial to the organism, something negative or life-ending to the organism, or nothing at all.