Every casino, no matter whether it is located in Las Vegas or on the internet, should boast a payout percentage. This is probably the single most important piece of information you can glean from an online casino, because it will tell you at a single glance exactly how generous and fair it is. In fact, most licensed and thus regulated online casinos are required (under the laws, rules and regulations of the jurisdictions in which they are licensed) to publish their payout percentages.
A casino payout percentage is simply the proportion of money (which is obviously expressed as a percentage) that is paid out to players in the long term. Take, for instance, an online casino that lists a payout percentage of 97%. This means that 97c of every $1 wagered is paid out to players as winnings, while 3c is kept by the casino. Payout percentages are usually updated every month and are legally required to be verified by third party auditing firms such as PricewaterhouseCoopers.
What's interesting is that the payout percentages offered by online casinos are usually much more generous than those offered by their land casino counterparts. The reason for this is that land casinos have enormous overheads (buildings, staff and maintenance costs etc) while online casinos, which are operated from safe and secure servers around the world, have far smaller overheads. This allows them to pass on the savings to players by offering more generous percentage payouts.
When you want to find out what an online casino's payout percentage is, it will usually be located on the casino's homepage or somewhere under or near a 'Fair Gaming' link. You'll notice that payout percentages are usually broken down by game (slots, table games, card games, video poker games, fixed odds etc) as well as expressed as an overall or total percentage. The rule of thumb is that any online casino that does not publish its payout percentage is not worth playing on.