Kickers
Though they seem like an insignificant part of a hand, one
important thing to pay attention to in Texas Hold'em are your
kicker cards.
Kicker cards are defined as those cards which do not directly
contribute to the main type of your hand. For example if you
have three-of-a-kind queens, the other two cards would be your
kickers.
There are many situations where the quality of your kicker
means the difference between winning and losing a hand.
If your hand and an opponent's are equal except for a kicker,
then the winning hand is said to "out-kick" the losing hand.
Here are some common situations in Texas Hold'em where kickers
may be important in determining who wins or loses a hand:
Example #1:
In this hand, there are four Sevens on the board giving both
players a Four of a Kind, Sevens. Player 1's kicker card
is an Ace, whereas player 2's kicker card is only a King.
As a result, player 1's hand is better and since it
came down to the players' kickers to decide the
winner, player 1 is said to out-kick player 2.
Example #2:
This hand is the same as the above, but each player's
kicker card is an Ace. This means both player's
have the same hand: Four of a Kind, Sevens, Ace kicker.
It was kind of unfortunate for player 1 that the fourth
seven came on the turn. Using
Poker Academy's Showdown Calculator, you can see that
on the flop Player 1 had a 66.7% chance of winning before
that happened.

Example #3:
In this hand, each player makes a pair of Kings.
However, player 1 has the better hand
since their Ace out-kicks player 2's Queen.
Example #4:
In this hand, each player makes a pair of Kings.
Each player's hand is equal up until their fifth and final
card where player 1's Ten out-kicks player 2's Nine, giving
player 1 the better hand.