It's a Head-Up Display, or HUD.
The HUD can display a number of different pieces of information, including performance data such as airspeed, altitude, and attitude, navigation data such as waypoints or ILS glideslope, or weapons data such as the selected weapon or the targeting reticle you mentioned. Thus, its uses are varied - depending on how sophisticated the system, basic aircraft control may be accomplished using only the HUD. In some modern fighters, there may be additional data on a Helmet Display (or in the case of the F-35, the helmet may have the only display).
It is not just used on fighters - some airliners, cargo planes, and trainers have HUDs:
As for your last question, per fooot's comment, this question and answer has more information.
The Blackburn Buccaneer was the first production aircraft with a HUD and entered service in 1963.