At any online casino almost all games are available in free practice mode. Don't deposit money as soon as you register. Demo mode would be more than enough at first. It can help you play slot machines casino online has.
Unfortunately, some of online casino games require real money bets. These are first of all progressive jackpot slots. It seems like this category of online slots is treated differently by online casinos. Progressive slots are offered as a premium product players should pay for. Perhaps that's because online casino administration tries to get customers interested in real money bets. Usually bet limits of progressive jackpots are a bit higher than limits of standard slot machines.
Live casino games are also unavailable in practice mode. No casino online would allow to play games with real dealers for free. Professional dealer services are quite expensive to let players pay no price.
Thanks to practice mode, online casino player gets the opportunity to learn rules of a selected casino game and at the same time see how interesting this game is. Free games don't have any restrictions regarding how long you can play or how many virtual chips you can lose. There are no rule differences compared to standard play mode. But you won't be able to win real money.
Many online casino software developers offer free versions on their websites. This is done not really for players, but first of all for potential clients which are casino online operators.
In contrast to online casinos where free mode is often interrupted by promotional banners, developers don't practice anything like that. Moreover, usually you can find the most recent casino games there. On developer's website you don't need to register, while some online casinos require registration even to play in practice mode.