I'm an American so I'll give you my informed American opinion on it...
...there is only ONE difference between a "professional" and an "amateur": it is a TAX classification.
First of all, the US Tax Code states that the one requirement to classify yourself as a "Professional Gambler" is that you have no other primary income. This means that if you work a full-time day job, you CANNOT file your taxes as a professional. Even if you are the greatest poker player in the world, you will only qualify as an amateur.
The main difference in this classification is that professionals get special tax treatment. It gets technical to explain, but professionals can deduct their losses against their winnings. Amateurs cannot do this, which means amateurs get screwed by the US Tax Code.
Let's use two examples, two identical players except one is able to call themselves a "Professional" but the other works at a factory and can only file as an "Amateur".
Each player has a break-even year. They both have significant tournament wins during the year, but those winnings are offset by their losses. They both enter $100K in tournament buy-ins during the calendar year, and they both win exactly $100K in payouts. Thus, they each "net" zero income - $100K in winnings with $100K in losses.
The Professional can sum these up on their tax return, and end up paying zero in tax to the government. The Pro has no income which means no tax is assessed.
However, the Amateur is not able to do this. The Amateur must claim the $100K they won as "income" and pay the full tax on that income (generally from 25% to 33%, so they owe the government anywhere from $25K to $33K). Amateurs cannot deduct losses from these totals, except for the very small "standard deduction" which is almost insignificant. Amateurs must add their poker winnings to their income from their day job and pay tax on that entire amount.
Disclaimer: I am not providing tax advice, some of my information here may be incorrect so do your own research. However, pre-Black-Friday (April 15, 2011), I held a full-time day job and thus I had to file as an amateur poker player so I learned the hard way just how terrible the tax code is on us. I actually quit my "real job" to play poker full-time as a "Professional" so I could obtain the tax advantage for that status... and believe it or not, my last day on the job was April 2011... after the government took away our freedom to play
online poker that fateful day, I never made any income from poker, so I never actually became a professional.
These days, I call myself a "play chip pro" which is a comical joke about this whole subject. I've been angry for the past decade about how they stole my dreams, but I did have the option to leave America and move somewhere I could play poker and follow my dreams... I chose not to do so, and still live here despite it all. I grind the play chips on
poker stars and rose to the highest ranks there. At least I became a baller somewhere, even if its more of a joke than anything. I like to poke fun at the whole thing, its maddening of course but I'm not gonna give myself an ulcer and ruin my health over it. It is what it is, this is our world, its not fair but oh well... and so it goes. LOL
Hope this explanation helps.