Short answers, pulled from the story.
The Maginot Line was built by France in the 1930s to deter a German invasion by making a direct attack so costly that Germany would be forced to enter through Belgium instead, where French forces would engage them. It was also designed to protect the industrial resources of Alsace and Lorraine, buy time for French mobilisation, and economise on manpower given France's population of 39-40 million versus Germany's 70 million.
Construction of the Maginot Line cost around 3 billion French francs, equivalent to approximately 3.9 billion US dollars. Work began in 1929 and the main construction was largely completed by 1939.
Germany used the Manstein plan, which sent armoured forces through the Ardennes Forest north of the main fortifications rather than assaulting them directly. German Army Group C facing the line held only 19 divisions, fewer than one-seventh of the total German invasion force, while France had committed about 36 divisions to defend the line.
Historians disagree. The fortifications themselves were never breached by a frontal assault; the main structures withstood aerial and artillery bombardment, and most remained intact when the armistice was signed on the 22nd of June 1940. The defeat came from Germany bypassing the line through the Ardennes and from failures of French strategy, communications, and response speed rather than from any collapse of the fortifications.
France re-manned and modified the line after the war, but the arrival of French nuclear weapons in the early 1960s made it an anachronism. When France withdrew from NATO's military component in 1966, most of the line was abandoned, auctioned off, or left to decay. Some ouvrages were converted to wine cellars or mushroom farms, and Ouvrage Hochwald remains active today as a hardened command facility for the French Air Force.
The Maginot Line was 20-25 kilometres deep and contained 142 ouvrages, 352 casemates, 78 shelters, 17 observatories, and over 100 kilometres of tunnels. It ranged from border blockhouses camouflaged as homes to large gros ouvrages housing 500 to more than 1,000 soldiers, connected by narrow-gauge electric railways and equipped with power stations, ventilation systems, mess halls, and water storage.