When you enter a showdown, the ranking of each player's hand is used to decide who wins the pot. Therefore, it is very important to understand how poker hands are made and ranked.
In Texas Hold'em, a poker hand is always the highest ranked five-card hand that can be made using any five card combination out the seven cards available to each player. The seven cards available to each player include their two private hole cards and the five public community (or board) cards that all players share. To put this another way, a poker hand can contain zero, one, or both of a player's hole cards. In the case where a player is using none of their hole cards, their entire poker hand is defined solely by the five public community cards, and this is commonly called "playing the board".
Poker hands can be broadly categorized into types according to the standard five-card poker hand types. Each of these hand types can be used to give a rough ordering for poker hands: all hands belonging to a particular hand type are ranked higher than all hands corresponding to types below it, and are ranked lower than all hands corresponding to types above it. For example, any Three of a Kind beats any Two Pair, but loses to any Straight. The relative ranking of these types is based on the relative likelihood of a hand of that type occurring when considering all possible five card combinations out of a standard deck of 52 cards.
In a standard deck of 52 cards, there are four suits: Spades, Diamonds, Clubs, and Hearts. Within every suit there are 13 ranks which are ordered from highest to lowest as Ace, King, Queen, Jack, Ten, Nine, Eight, ... , Two.
In Texas Hold'em, unlike some other card games, all suits are considered equal and are not used to break ties in hands that are equal except for suit differences.
For hands of the same hand type, the ranking of the hands within a hand type depends on the ranks of the cards making up the hands.