What is Gene Therapy?

Gene therapy is a practice of inserting, alternating or removing genes in a living creature which may cause disease or other problems to its body. The most usual form of gene therapy includes inserting new functional genes into a genomic location to replace a mutated or in other words, faulty and unwanted gene. The other option is simply to directly correct the mutation or modifying other genes to help alter it. Gene therapy can also be used in allowing people to choose traits for which their children can inhabit to prevent disorders usually linked to the X chromosome or for parental preference over smaller things such as the phenotype of their offspring.
Gene therapy is much harder than modifying bacteria though because of problems involved in carrying large sections of DNA and delivering them to the correct site on a gene. 

Even though gene therapy is not a very widely used practice and is in its early years, it has had some successes.

There are two main types of gene therapy:

Germ line gene therapy:
This is where reproductive sex cells are introduced with functional genes incorporated into their genomes (a full set of chromosomes of genetic information inherited by parents). Hence, the changes would be heritable and passed onto later generations. It is highly effective in countering genetic disorders and hereditary diseases but is not legal for now.

Somatic gene therapy:
This form of gene therapy has therapeutic genes or genes which can help treat disease/disorder transferred into somatic cells of a patient. Somatic cells are every cell in the body excluding sex cells. Thus, any modifications and effects are restricted to the individual patient and cannot be inherited.

Scientific Principles
There are three steps involved in gene therapy:

Step 1: Partial removal of patients cells
Step 2: Introduction of normal, functional copies of the gene through vectors
Step 3: The reintroduction of the modified cells once the genes have been fixed

In order to execute gene therapy a technology is used to carry a selected sequence of DNA and transfer it into another cell. These are called vectors. 

There are three types of vectors: Viral, Non Viral and Hybrid 

Viral: This technique basically copies how viruses spread in the body with the only difference being that the virus is good rather than bad. These viruses bind to their hosts and introduce genetic material into the cell as part of the replication cycle. This genetic structure contains instructions to produce more replicates of the good virus to create functional proteins. 
There are different types of transfer methods for different areas of gene therapy, some of these include:
Retroviruses: This form of virus helps introduce RNA and enzymes in a person's genes
Adenoviruses: These are used to carry genetic information in the form of double-stranded DNA and cure respiratory, intestinal, and eye infections in humans
Adeno-associated viruses: Used for small viruses with a genome of single stranded DNA and is often used for chromosome 19
Herpes Simplex Virus: This virus relates to gene transfers within the nervous system. 

Non-Viral: Is any process which does not involve a virus. This method has firm advantages over viral methods. These include low chances of provoking the immune system and a larger scale for gene production.
Its disadvantages include low gene expression where gene information is used in the combination of a functional protein. Another problem Non-Viral vectors had was low amount of nucleic acid introduced into cells, but has been fixed over the years.
Some examples on Non-viral transfers include:Electroporation: A method that uses short pulses of high voltage to carry DNA across a cell membrane
Gene gun: Another physical method of DNA transfection (introducing nucleic acid into cells) which generates a force to penetrate this into cells
Sonoporation: This uses ultrasonic frequencies to add DNA into cells by disrupting the cell membrane and allowing DNA to move into cells
Magnetofection: with this method, DNA is made into magnetic particles and placed underneath tissue to bring the DNA into contact with a cell monolayer (a layer of cells where no cell is growing over another)
Non-viral  can also include chemical techniques such as Oligonucleotides.
Hybrid: Obvious from its name, this technique combines the methods of gene transfer due to the fact that every method has some downfall. 

Fun fact: Gene therapy can also restore muscle to people which has been lost through age or disease, but athletes are eyeing this as a way to improve their performance.  This is called Gene Doping. 

Diagram 1

As seen in the diagram above, a genome enters a host cell and replicates. Its code for creating proteins is processed and then passed on to other cells to repeat the process. 

Diagram 2 

This diagram outlines a Viral vector being executed. It forms with the cell membrane,breaks down as it enters the cell and makes protein from the new gene.