DNA is an abbreviation of the chemical name: Deoxyribonucleic acid.

The building blocks of DNA are called nucleotides. All nucleotides have three basic components: a phosphate group (negatively charged), a pentose sugar (pentose meaning it has five carbons) and an organic ‘base’ with has nitrogen in it, often referred to as a nitrogenous base. 

There are 2 groups (ie. types) of nitrogen containing bases in nucleotides. These are pyrimidines and purines. In simple terms; think of these both as being based on rings of 4 carbons with two nitrogen atoms - with other atoms attached to various parts of the rings. 

Pyrimidines have one ring in their structure. They include cytosine, thymine and uracil.
Purines have two rings in their structure. They include adenine and guanine.

James Watson and Francis Crick established that a phosphate from one nucleotide attaches to the sugar in the next nucleotide. When long strands are formed in this manner, it is referred to the sugar phosphate backbone. In DNA the sugar used is deoxyribose (a sugar derived from ribose with lost oxygen). 

The ribose sugar has hydrogen in place of a second hydroxyl (OH) group. This means that individual deoxyribonucleotides can bind to each other by their ribose residues. This forms the ‘backbone’ of DNA molecules. Protruding out from the sugar phosphate backbone are the various bases arranged into a particular sequence. It is the sequence in the bases (A,C,T,G) which determine the genetic code. 

Double Stranded Helix 

DNA is called a double helix. DNA is double stranded because the bases can bind to each other, purines bind pyrimidines and pyrimidines bind purines. The result is a double helical structure with the two sugar backbones on the outside and the bases are on the inside.

Therefore DNA is made up of two strands, being held by hydrogen bonds between the nitrogenous bases.  The nucleotides join following a set of rules called base pairing rules. That is adenine always pairs with thiamine and cytosine always pairs with guanine by either two or three hydrogen bonds respectively. These hold the two strands together. 

On one strand the phosphate is positioned in an upwards position, whereas on the other strand the phosphate is pointing down. This opposite orientation is called antiparallel which means the arte aligned in opposite directions. This overall structure forces the two strands to coil around each other in a corkscrew (helix) formation. This is the DNA double (two strands) helix.  This becomes a stable structure.