These are the four standard nucleotide bases adenine (A), thymine (T), guanine (G), and cytosine (C). Here, they are all attached to sugar and phosphate (on the left, the violet "star" is phosphor). Sugar and phosphate can be bonded ("glued") together. This makes the assembly of the nucleotides into a long strand possible. The strand has a backbone made of sugar and phosphate, and the nucleotide bases are like washed laundry on a clothes-line, as below. Finally, two such lines can build a double helix, as shown in our first figure, and the background image.
If you have a VRML viewer installed, click on any nucleotide and you will get a 3-dimensional "hands-on" experience. Similar ball-and-stick models can be constructed for the 20 amino acids.
The nucleotide pictures were taken from the Image Library of Biological Macromolecules. (See also Jürgen Sühnel, Image Library of Biological Macromolecules, Comput. Appl. Biosci. 1996, 12, 227-229.)