The default behavior for anchor tags when no target is specified is to open in the same window. _blank will open a new window, except in some buggy versions of Internet Explorer 5 and 6. Your browser should be ignoring all other target names.
6.16 Frame target names
Except for the reserved names listed below, frame target names (%FrameTarget; in the DTD) must begin with an alphabetic character (a-zA-Z). User agents should ignore all other target names.
The following target names are reserved and have special meanings.
_blank
The user agent should load the designated document in a new, unnamed window.
_self
The user agent should load the document in the same frame as the element that refers to this target.
_parent
The user agent should load the document into the immediate FRAMESET parent of the current frame. This value is equivalent to _self if the current frame has no parent.
_top
The user agent should load the document into the full, original window (thus canceling all other frames). This value is equivalent to _self if the current frame has no parent.
http://www.w3.org/TR/html4/types.html#h-6.16