Judaism strongly disapproves of gambling in general, and betting in particular.
The Talmud (Sanhedrin 24b) disqualified gamblers from being witnesses or judges on two grounds: quasi-theft, and uselessness. Quasi-theft because each side to a bet hopes to win, and that hope taints their consent to the transaction; taking people's money by exploiting their unrealistic expectations is not so very far distanced ethically from taking it from them by fraud (or even violence). And uselessness because Judaism teaches the importance of each person trying to earn a livelihood by contributing something useful to the world; taking other people's money through gambling contributes nothing, and is at worst dishonest and at best parasitic.
Dealing on the stock market need not fall foul of either of these objections. Investing in shares enables businesses to raise working finance: the highest form of tzedakah (charity) is lending people money so they become self-sufficient and retain their self-respect, and venture capital can be a similar ethical proposition.
As to other kinds of dealing, the ethics depend on the circumstances. If I sold shares and you buy, simply because the present needs of our respective investment portfolios are different, there is no problem; but if we trade because each of us thinks we know better than the other, our actions are, again, exploitative at worst and parasitic at best.
