The F117 is a stealth aircraft designed for the USAF by Lockheed. It is commonly referred to as the "Stealth Fighter." This is inaccurate. It actually fills the roles of ground attack aircraft and reconnaissance aircraft. It routinely flew reconnaissance missions over the Eastern Bloc countries during the 1980s.
Because of its odd design (which is not very aerodynamic), it uses a fly-by-wire system; were it not so, the pilot would not be able to fly it at all. The aircraft uses a number of technologies to gain stealthiness:
The materials used in the construction absorb some of the RADAR energy
The shape of the aircraft reflects most of the rest of the RADAR energy away from the source
The aircraft is painted with a color called Cannonball Black. This makes it very difficult to see it at night.
The engine housings are shielded to greatly reduce the IR signature of the aircraft, making that form of attack difficult.
The F117 does not carry any RADAR equipment itself. Instead, it uses an FLIR to target. It is capable of dropping self-guided RADAR missiles, however.
Despite all of these methods, the aircraft is still somewhat visible to RADAR; however, F117 pilots are trained to fly close to the terrain (below 400 ft.) which allows them to blend in to the background noise. They also use special techniques like "threading the needle" which involves knowing the location, type, and range of all RADAR sites in the area and taking a carefully planned route that winds in between the coverage of the sites. Even if you can't find a blind spot to fly through, you can also fly in a circle around the site to further minimize the amount of useful data returned to the RADAR array. The visibility to RADAR increases when engaged in banking or other maneuvers. The point of greatest RADAR visibility is when the bay doors are open.