A general strike is a strike by a wide range of workers. It's contrasted to a regular strike, which is usually a single union striking against a single employer or company. A general strike involves multiple unions and people who are not in unions.
General strikes were outlawed by Taft-Hartley, which redefined a strike to limit it to one workplace. Labor laws gave special protections to strikers, but then withdrew rights to sympathy strikes, which are strikes carried out in sympathy with another strike.
The target of a general strike is usually regional, in a city. The Oakland general strike's goal is to shut down the downtown, so no business can happen.
There are also general strikes within an industry, like in logistics, in Hollywood, or other industries where there are multiple unions. Generally, these have been disallowed in the contracts that the unions have negotiated.
The goals of the general strike vary, but one usually unstated goal is that the general strike can become an exercise in preparing for larger social changes. During a strike, committees may be set up to provide food.
This is similar to the social infrastructure that Occupy Oakland set up at Oscar Grant Plaza. The idea is to form rudimentary structures for survival to provide food, clothing, shelter, and other necessities.