Alternating Current (AC) flows one way, then the other way, continually reversing direction whereas Direct current (DC) always flows in the same direction,
So AC doesn't have a plus and minus, the + and - reverse 50times a second (in the UK as we have 50hz mains)
power stations typically have turbo-alternators that are just known as "generators", car alternators have a rectifying circuit in them so they ultimately put out DC, but they initially produce AC.