According to the I.R.S. Tax Code, casino winnings are, like any income, taxable.
If you buy a lottery ticket for a dollar (or more) and you "win" $10, that is (supposedly) taxable.
The question is how can one earn 40K in a year and keep it.
In a casino there are no taxes withheld - except for slot machine winnings in excess of a certain amount on a single play (I forget. It is either $1,200 or $2,000) and for tournament prizes equal to or in excess of $5,000.
At the racetrack if I bet $10,000 on a 10-1 shot and win $100,000 that much cash will be handed to me - without any tax being withheld. However if you buy a $2 Trifecta ticket and it pays $750 you will need to show I.D. and fill out a tax form in order to cash that ticket - BECAUSE your ticket meets BOTH of the following ... The odds were 300-1 or more AND you won OVER $600. My win was at odds of 10-1.
If you go to a casino and play BJ and win lets say $1,500 do you think that you have to show identification? NO. In some venues such as Las Vegas, casinos will follow a state protocol that requires the casino (not the I R S) to ask for your proof of identity if you wish to cash in over $3,000 in chips.
Anyone who has casino CASH transaction in excess of $10,000 in a 24 hour "gaming day" will have a C T R (Cash Transaction Report) sent to the Treasury Dept. (and C.C.'d to the I.R.S.) in order to dissuade money launderers and drug dealers from practicing their nefarious trades. Anyone handling his cash transaction in order to avoid handling that much cash within the set 24 hrs. is guilty of a federal offense called "structuring"
Anyone buying lots of chips and cashing them out without gambling very much or gambling in such a way as to circulate the currency without actually risking much money at all will generate a "Suspicious Activity Report" and that too is sent (without your knowledge) to the Treasury Dept.
The I.R.S. will permit one to function as a professional gambler (like one of the expert Poker players one may see on televised tournaments). The only unfair aspect to the tax code is that the professional gambler is the ONLY business person who cannot carry over his losses from one year to the next. Ergo, if a pro gambler WINS $5,000 on 12/31/09 and continues to play and is unlucky enough to lose it back - breaking even AFTER midnight, they treat the gambler AS IF he won $5,000 instead of breaking even. The loss is of no concern to the I R S If for the year 2009 the gambler shows a loss and in 2010 shows a modest profit, he will be fully taxed on his 2010 earnings without being able to write off his 2009 losses. The I.R.S. has never been able OR willing to explain how this is a just practice.
I hope that this post answers your question.