Cash games are regular ring games which lack the tournaments’ blind structure and thus place less pressure on players to act.
In a tournament, the blinds increase periodically compelling players to act, and sometimes to act in negative EV situations. In a cash game, this pressure doesn’t exist, thus people can play more relaxed. What does this mean to you? Tighter and more difficult-to-beat games possibly, and a constant trickle of rake.
In a cash-game, you pay rake on every single pot that is won. No matter who wins the pot, if you contribute to it, you’ll be paying part of the rake. This places a different kind of pressure on cash-game players: the pressure to win. If you go winless for a long time not only do you not pocket any money, you spend quite a lot on blinds and the rake. Remember that this is real money at stake here and not symbolic poker chips.
On the other hand, cash games offer instant gratification: whenever you take down a pot, what you win is all real money and is all yours on the spot.
Even though most professional poker players are known for their tournament antics, they do play cash games as well. Watching shows like High Stakes Poker is potentially more entertaining for the public than watching tourneys, because the currency that changes hands is real, and people can truly relate to the piles of banknotes heading into the winner’s stack.
This kind of poker requires some extra-sharp skills, and – in NL events – fortunes can change hands at the flip of a card on the river.