Most gambling systems rely heavily on prediction and likely odds.  The odds change constantly in a game like blackjack or poker because the deck changing.  If you're looking at how to win at online roulette then you'll possibly come across the very plausible system called Martingales.    This system was first used in the 18th century in Paris - I'm surprised all the casinos aren't bankrupt by now!  It's a very simple strategy that involves betting on an outside bet like red/black and doubling your stake each time you lose.  So if you bet $1 on red and it loses you  then bet $2,$4, $8 etc until you win.  The idea is that you will eventually win and you can only lose by not having enough money to cover your stake requirements.    

Now it all sound very plausible except the concept it is based on is known as the 'Gamblers Fallacy' - a widely documented mathematical fallacy.   The very simple facts are that the odds of red or black coming up in our example are not influenced in anyway by previous spins of the wheel - if you get 14 reds in a row - the chance of another red coming next are still 50/50 (although on a roulette wheel - the 0 makes this not quite correct).  The problem with the Martingale system in practice is that it can get very expensive very quickly as you will find out if you try it and you get red 8 times in a row!  

You'll also find that online casinos often have a limit to identical bets to discourage this sort of gambling - although most don't simply because they make money out of the Martingale system. I think the main reason we can know that this system doesn't work is that it has been talked and tested by gamblers over hundreds of years and still the real and online casinos make money.   You'll still see Martingale system appearing in web sites and ebooks as a 'secret system' - though  not a terribly well kept secret.   When you see someone trying to make it work it gets kind of sad - I have seen someone pull out after 11 consecutive black spins - not sure if he lost his bottle or simply ran out funds but he lost some serious money.