In Texas Hold'em you get two cards that are your own (hole cards). You combine them with the common cards (community cards) to make a 5 card poker hand.
You can use both your hole cards together with 3 community cards, or one hole card and 4 community cards, or all 5 community cards. In the last case, the best you can hope for is to split the pot.

You choose this based on what gives you the best poker hands. In online poker this is done automatically, but you still have to understand it of course.

Below is the best poker hands ranking from the top and downwards. In poker all suits are ranked equally.

Royal flush

A straight from a ten to an ace with all five cards of the same suit.

Straight flush

Any straight with all five cards of the same suit.

Four of a kind

Any four cards of the same rank.

Full house

Any three cards of the same rank together with any two cards of the same rank. If several players have a full house the player with the highest rank of the three cards with same rank wins. Our example shows "Aces full of Kings" and it is a bigger full house than "Kings full of Aces."

Flush

Any five cards of the same suit (not consecutive). The highest card of the five determines the rank of the flush. Our example shows an Ace-high flush.

Straight

Any five consecutive cards of different suits. Aces can count as either a high or a low card. Our example shows a five-high straight, which is the lowest possible straight.

Three of a kind

Any three cards of the same rank. Our example shows three-of-a-kind Aces, with a King and a Queen as side cards - the best possible three of a kind.

Two pair

Any two cards of the same rank together with another two cards of the same rank. The highest pair of the two determines the rank of the two-pair.

One pair

Any two cards of the same rank.

High card

Any hand not in the above-mentioned hands. A hand with for example a 9 as highest card is called a 9-high. Our example shows the best possible high-card hand.