Here’s what you need to know about the U.A.W. strike.

Why is General Motors a target?

G.M. has a smaller U.A.W. work force than its Detroit rivals, Ford Motor and Fiat Chrysler. But the union has taken aim at G.M. as the automaker has earned solid profits — it made $35 billion in North America over the last three years — while closing auto plants in the United States.

One of the union’s objectives is getting G.M. to reopen a shuttered car factory in Lordstown, Ohio, a goal that President Trump has endorsed. G.M. closed that plant, and others in Baltimore and in Warren, Mich., as part of a cost-cutting effort that eliminated 2,800 factory jobs and thousands of white-collar positions. A new contract could also decide the fate of a plant in Detroit that G.M. has kept open after designating it for closing.

Focusing on a single company is a standard practice in the talks between the U.A.W. and the Detroit automakers every four years. For now, the contracts with Ford and Fiat Chrysler have been extended.

How big an issue is pay?

One sticking point in the negotiations is the automaker’s tiered wage structure — workers who have been with G.M. since before 2007 earn about $31 an hour, most of those hired since then make much less, and so-called temporary workers are at the bottom of the wage scale at about $15 an hour. Benefits packages also vary.