I will start with the beginning. You may ask what is card counting? Well, card counting is a game strategy (used primarily at blackjack) to determine when the next hand gives an advantage to the player. When there is player advantage bets should be considerably increased.
For blackjack there are many card counting strategies (some easier to learn, some harder) like Hi-Lo (the most known one, and probably one of the easiest to learn), Kiss, K-O, Uston, Reko ,Omega, Revere, Canfield, Mentor, Zen, etc. Which is the most efficient? -- That's another discussion but I'd argue you should review all and go with the easiest for you (preferably not Hi-Lo since is the most known by casinos too).
Also, under certain circumstances card counting might help you win at other games too, like for example if you find a version of baccarat, like EZ baccarat (or No Commission Baccarat) -- not necessarily online -- which offer the Dragon 7 (banker wins with a 3 card for a total of 7 which pays 40/1) and Panda 8 (player wins with a 3 card for a total of 8 which pays 25/1) side bets.
A warning word though: card counting is not welcomed by casinos (neither offline or online). In a land based if you get caught you'll be asked to flat bet or leave the game (worst scenario would be to be banned from entering the casino again) -- However, you will be paid your winnings since card counting is legal! On the other hand, if you want to pull this strategy online, you should know that some (not all!) casinos expressly prohibit card counting (you should also watch out for "professional players / bettors" expression) in their Terms of Service, and if you get caught you might very well risk losing all your winnings, including your deposits.
In another order of ideas, do take note that perfect card counting in your own home is much easier to do (there are even tools to help you!) than doing it in a brick and mortar casino where there are lots of distractions and where you also need to mimic, smile and talk with the dealers without losing the count. -- You can't tell the dealer "don't disturb, I'm counting"! Really, not many people have these skills...
Now a few words about the profitability of card counting (at blackjack): to get the most of it, you should bet only when the odds are in your favor --this is called wonging-- technically this is not possible online because you will lose your seat after two hands without a bet. What you can do is, do a half-wonging (ie. when the count is negative play only every other hand). And you should also look for tables with the highest penetration into the shoe before shuffling! This is very important because the fewer cards are left in the deck, the more efficient will prove to be your count. For example a double deck shoe with 75% penetration before shuffle is way better than a single deck shoe with 50% penetration! (But I'm talking non-sense here since you'll hardly find such games offline. And online are all 6 or 8 decks with 50% penetration -- and yeah, for these to beat and be worthwhile you need to bet with a 1 - 50 unit spread (starting bet $10, max bet $500).