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In Texas Hold'em, there are 5 communal cards that are shared by all players, and 2 hole cards, that are private and may only be used by the player that the cards were dealt to.
Can we just select all the 5 communal cards for creating our best hand, or do we need to keep our 2 hole cards and only choose 3 communal cards out of 5 communal cards to make a hand?
In Hold'em, each player makes the best possible 5-card hand possible using any of the 7 cards available. To each player, there is no distinction between the communal cards and their own private cards. The selected 5-card poker hand can include 0, 1, or 2 of the hole cards.
It's possible that the best hand you can make uses 0 of your hole cards. In this case, you cannot win (unless everyone else folds), but if no player can make a better hand than the 5 communal cards then it is a tie.
Poker hands, that is, sets of cards that can be ranked by standard poker rules, contain 5 cards. Many poker variants, including Texas Hold'em, give players access to more than 5 cards and let them create the best 5-card hand they can out of all the options. In 7-card stud, some of the cards are private, but face-up. In Texas Hold'em, some of the cards are shared.
As you said in your question, in Hold'em there are 5 communal cards and 2 hole cards, so each player has 7 cards from which to choose 5 to make the best poker hand they can. The number of ways to choose 5 from 7 is 21 (Combinations, from Wikipedia).
You might think you can get away with omitting the combination consisting of just the communal cards, but this should still be evaluated because if it's the highest scoring hand, then everyone ties (e.g., if the communal cards are a Royal Flush, then everyone ties regardless of their hole cards).
shujaa's answer is completely correct for Texas Hold'em.
There are other poker games where you are required to use a certain number of hole and community cards. For example, Omaha Hold'em is dealt similarly to Texas Hold'em, but each player gets 4 hole cards instead of 2; and in the showdown, your hand must use exactly two of your hole cards and three community cards.
