target=_self

This will load the page in the same frame as the caller.
  表示在目前窗口或框架里打开链接（默认的）

target=_parent

This causes the page to load in the same frameset as
the caller taking the place of any sub frames.
  表示在父窗口或框架里打开链接

target=_top

This loads the resulting page into the full body of
the current window eliminating any frames.
  表示在顶层框架里打开新链接

target=_blank

This opens up a new window and loads the page in to it.
  表示始终在不同的新窗口打开链接

target=_new

Opens a new window and the page loads in it, all other links using same target open in this window.

表示始终在同一个新窗口打开链接