DNA is nothing more than a sequence of bases (nucleotides) and since DNA is normally double stranded, they can be referred to as base pairs. One could best visualise it like a zipper: two connected strands (that can also be separated). DNA is made up of only four different bases, abbreviated as A, C, G and T. These always form the same pairs: A on one side of the zipper, T on the other side and the same goes for C and G. So, when unzipped, you always know the sequence of the opposite strand.
The sequence of base pairs that make up our DNA should be viewed like a bar code. Every set of three bases code for one building block of a protein. That's all that DNA is for: code for building proteins. A set of three bases is called a codon and tells machinery in the cell (ribosome) to add one specific building block to a forming protein. It's like Lego and DNA is the instructions that tell you which block to add next.
These different blocks give shape and function to the proteins it helps to build.
