I know this matter pretty well because last 3 years I file tax return as a professional poker player. Recommended reading is the book by Ann-Margaret Jonston, CPA "How to Turn Your Poker Playing Into A Business"
1. You must pay taxes on all gambling (poker as well) winnings.
2. If you file as a professional gambler, you can treat your winnings as a profit from trade or business and can deduct your loses from your winnings. Also, you can deduct some gambling related expenses. Just like any other business.
3. To consider yourself as a "professional" you must meet certain IRS requirements, such as: playing regularly, exclusively for profit, spending considerable amount of time on this, and so forth.
4. Otherwise, if you are non-professional player, your must claim winnings form all your winning sessions as income, and report all your loses as itemized deduction on Schedule A. Session definition is pretty vague.
In some cases, your are going to pay additional taxes on gambling winnings, even though overall you lost money this year.
5. Also, you should take in account, that some states have different from IRS regulations regarding gambling income. For example, in Massachusetts
gambling loses are not deductible at all. So, if one day your won $1,000, and after, or before that you lost 10,000, you anyway have to pay taxes in full amount on that $1,000. Its weird, but it's so.
If you won in casino and they issued 1099G for you, I would strongly recomend to report it. It's up to you to report online winnings or not, since poker sites
don't cooperate with IRS. But if you regularly cashout big into your bank account, it may raise a flag. Banks do cooperate with IRS.