US Rules in Brief
All gambling winnings count as ordinary income. Books issue a W-2G for certain large wins (e.g. $600+ at 300:1 odds) and may withhold 24% on very large payouts, but the obligation exists regardless of paperwork. States add their own layer — some tax winnings heavily, a few not at all.
Deducting Losses
Losses are deductible only up to winnings, and only if you itemize deductions — taking the standard deduction means paying tax on gross wins while eating losses. Professional-gambler status changes the math (net income on Schedule C) but carries a high bar to claim.
This is general information, not tax advice; a CPA familiar with gambling income is worth their fee for any serious volume.
Records That Survive an Audit
Keep a contemporaneous log: date, book, market, stake, result, plus your annual win/loss statements from every book (all regulated apps provide them). Session-level records matter especially for casino play; bet-level exports cover sports.