Special goals.
There are 4 reserved target names to use for special document redirection operations:
_blank
The browser always loads the target document in a newly opened, unnamed window.
_self
The value of this target is the default target for all <a> tags that do not have a target specified, which causes the target document to be loaded and displayed in the same frame or window as the source document. This goal is superfluous and unnecessary unless used with the target property in the document title <base> tag.
_parent
This goal causes the document to be loaded into the parent window or the frameset that contains the frame referenced by the hyperlink. If the reference is in a window or in a top-level frame, then it is equivalent to the target _self.
_top
This goal causes the document to be loaded into the window containing the hyperlink, with the _top target clearing all the contained frames and loading the document into the entire browser window.
Tip: All 4 values of these target begin with an underscore. Any other window or target that starts with an underscore will be ignored by the browser, so do not underline the first character of any frame name or ID defined in the document.
From: http://www.w3school.com.cn/tags/att_a_target.asp
HTML a Tag target property function