They discovered that there's a whole lot of nucleotide pairs (3.2 billion) but very few genes (only 21,000). Scientists believe we make up for our lack of genes through alternative DNA splicing, a process that would explain our complexity. 
They also discovered that only a small portion of human DNA is for coding proteins.

A lot of it is noncoding DNA.  Scientists are largely unaware of what this "junk DNA" does.

Much of it is Repetitive DNA, nucleotide sequences present in many copies throughout the genome. 

Having  a mapped out genome is useful for diagnosing genetic diseases. 

Gene Therapy is the alteration of an afflicted individuals genes by replacing a defective gene with a normal allele. It is used to treat people with genetic disorders by repairing their defective genes by inserting normal alleles into somatic cells of the tissue that is being affected by the disorder. In order for the therapy to to fix the problem permanently the allele must be placed into cells that multiply a persons whole life. 

The procedure is:
1. A normal gene is cloned and inserted into a virus 
2. Defective bone marrow cells from the afflicted person are infected with the altered virus. 
3. The virus inserts inserts its DNA along with the new gene. 
4. The engineered cells are then injected back into the afflicted person and the normalized cells multiply throughout the persons life and cure the disorder.