Nucleotides are the building blocks of DNA.

The definition of DNA is: A nucleic acid that carries the genetic information in the cell and is capable of self-replication and synthesis of RNA. DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the complementary bases adenine and thymine or cytosine and guanine (the A,T,C,G you see in your results).
And more simply — The sequence of nucleotides determines individual hereditary characteristics.

There are 4 different nucleotides :

* dATP : deoxyadenosine triphosphat
* e dGTP : deoxyguanosine triphosphate
* dTTP : deoxythymidine triphosphate
* dCTP : deoxycytidine triphosphate

These are also know an (AKA) dNTPs (deoxynucleoside triphosphates).
A nucleotide is made of three major parts : a nitrogen base, a sugar molecule and a triphosphate. Only the nitrogen base is different in the 4 nucleotides.

DNA is formed by coupling the nucleotides between the phosphate group from a nucleotide (which is positioned on the 5th C-atom of the sugar molecule) with the hydroxyl on the 3rd C-atom on the sugar molecule of the previous nucleotide. To accomplish this, a diphosphate molecule is split off (and releases energy). This means that new nucleotides are always added on the 3′ side of the chain.

above two images courtesy of Andy Vierstraete at Avierstr