Online poker takes the concept of a card game played in a casino and cranks it up to the highest levels of competition you can imagine. You can play for nothing at all or for the smallest stakes imaginable, all the way up to satellite entries into the largest live tournaments in the world.
Online players can access a variety of tools to help them improve their games such as a hand history viewer and a range finder which helps you analyze spots in a hand. There is also software that allows you to run simulations and learn the odds of hitting a certain poker hand. This type of poker simulation tool is very powerful but it can take a lot of CPU power to run.
A series of events in 2011 put an extreme damper on the online poker marketplace. First, the UIGEA (Unlawful Internet Gambling Enforcement Act) was passed which banned payment processors from processing payments related to online gambling, effectively cutting off much of the money flow to regulated online poker sites. This led to several poker rooms folding, and it also made it more difficult for players to fund their accounts.
Since then, a number of states have regulated online poker and joined multi-state agreements to combine player pools. New Jersey, for example, was the third regulated poker market in the US, and it signed an agreement with Delaware and Nevada to combine their player pools in 2017. However, the DOJ has recently moved to reinterpret a wire act ruling that could put a crimp on this potential.