The building blocks of DNA are called nucleotides. The composition forms a winding step ladder called the double helix with a sugar and phosphate backbone and base pairs in the form purines and pyrimidine. Guainine (a purine) is paired with cytosine (a pyrimidine); and a second set adinine (a purine) is paired with thymine (a pyrimidine).
DNA makes proteins, but it also replicates itself by splitting the double helix down the middle to create two templates which then form the basis of a new double helix, and the process continues. To initiate the replication, the process occurs at certain locations of the DNA helix called origins and are then targeted by proteins to create the DNA synthesis. If you want to learn more about DNA replication, read the Bright Hub articles on the topic.