Card counting systems
Hi-low system
Counting cards in Black jack is very instrumental as it gives good information on the deck and its proportion on high and low cards. It also somehow implies a good option of hitting the blackjack. For example if there is higher number of higher cards to lower cards then you are closer to hit blackjack.
Also if there are more of lower cards then dealer is on the beneficial side. However more blackjack can hit the table with more number of 10s and more of aces and some successful double down. In the historic movie Rain Mann and other identical movies makes people believe that counting cards is possible but that is however not the case always. However heuristics can be used to track the total score and the count. There are several means like addition, subtraction and many other ways are used.
However the most popular and the easier method is the Hi-Low factor. In this ranking system, all the cards falling between 2-6 are considered as +1 while the cards with face and aces , and also the 10s are considered as -1. All other cards like 7, 8, 9 are considered as 0 since they are explained as neutral cards.
Other systems
Other than Hi low system, the methods use is Zen count. Here the values attaché to the cards are no longer +1 or -1, instead they use the values of +2 and -2 to distinguish more the playing cards. The intelligent players always keep a side counting of major cards. Like counting the aces can become instrumental at the time when there are important decisions taken on the game.
This however may be a disadvantage as it makes you slower while playing the game since you are side counting. The basic reason for this counting is to keep track on the 10 and aces if there is a disproportionate deck. The heavier the 10 and aces the more its worth betting. So to calculate your own risk better is counting. A simpler way to kill the issue is just to memorize what's been played and you know what's in the stack. However counting the cards never ensures a win every time so play safely.
Some casinos do object on card counting claiming it as illegal but to be honest, this is a major way to prevent cheating. The card counting policy has been there from ancient time and even today people try to use the strategy. But it's a belief of the player that counting gives a competitive mark to the game and thus they always vote for it. Some casino people and management are also there who encourages the policy. So count and enjoy the game.