Gene Therapy:
Gene Therapy is the attempt to treat a human by using their own cells with altered DNA as opposed to pharmaceutical drugs. The most common form involves using DNA which codes a functional, therapeutic gene to replace a mutated gene. 
The DNA that encodes a therapeutic protein is packaged inside a vector. A vector is an object which can help to deliver the DNA that has decided to be encoded, into the cell, usually an inactive virus that has been genetically modified to not cause disease but still enter human cells. Some types of virus, such as retroviruses, integrate their genetic material (including the new gene) into a chromosome in the human cell. Other viruses, such as adenoviruses, introduce their DNA into the nucleus of the cell, but the DNA is not integrated into a chromosome.
There are two methods of inserting the altered cells into the body, the first is that the adapted vectors can be introduced intravenously or alternatively a tissue sample can be removed from the desired area and the cells "occupied" by the vectors can be reintroduced into the body.
Once inside, the DNA becomes expressed and the therapeutic protein begins being produced by the cell, effectively treating the bodies own cells.
Technically gene therapy is still in an experimental stage but in 2008 it was seen to have the potential to "reverse" blindness. The video below is from the article that was written on this event 6 years ago. After success` on dogs and primates was seen, the procedure was allowed to be undergone on patients. A harmless virus called adeno-associated virus injected a working copy of RPE65 to the patients' retinal cells, allowing them to become functional to a degree.
I am very excited about gene therapy and although it is succeeding to only certain degrees at the moment ( for example in this trial only 4 cases were seen to be significantly improved) the potential is amazing, the ability to alter DNA for medical purposes is astounding. Imagine being able to alleviate or even cure genetic diseases. Some people are worried about the ethical implications though as altering human makeup is sometimes seen as against human nature.