what is stock?

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I have a heard about stock.what is it?please explain me in brief.I wanna make a small investment and earn some pocket money per month.Is it possible to do so?How much must i invest to get about 30$-40$ a month?I don't know anything about it.So please explain in brief and suggest me where to invest.Is there any other ways?I mean investing a little and doing them some work like pay/click or reading emails..