A "Flanking" maneuver is an attack on the sides (flank) or rear of an enemy force. It is usually initiated by a flanking force of fast moving units that can wrap around (pinch) enemy forces from the side, thus splitting the main force - and hopefully the attention of the defender. Generally the sides and rear of a force are not as prepared for a full-on attack so the flank/rear makes a good target.
"Flanked" and "Outflanked" both describe the condition of falling victim of a flanking maneuver. "I was outflanked" means the enemy wrapped around and attacked my flank and either I wasn't prepared and/or couldn't adapt to the attack quickly enough.