What is, and what are the different forms of, narrative voice?

Narrative voice is the perspective from which the writer has chosen to construct their work. Although the reasons why this choice was made may not be obvious, it is necessary to first identify what the voice is and work through reasoning as to why it has been written this way.
Firstly, narrative voice can take the following forms: First person, second person, third person, and third person omniscient. First person - use of personal pronouns such as I, my, we etc. A character within the story is the one telling it. Second person - use of personal pronouns such as you, your. The story is told from the perspective of the reader. Third person/third person omniscient - use of personal pronouns such as he, she, they etc. The story is told from a perspective outside of the characters. It may be omniscient (the perspective knows more than the characters do), or only be as informed as the characters themselves.
When considering why a narrative voice has been chosen it is important to consider: how the reader feels about the narrator and the characters, what the intended relationship is between the reader and narrator, and the general aims of the specific narrative voice and how they apply in the context.