The one decision made by every poker player on every deal of every game is whether or not to continue playing that hand after seeing that first set of cards. Since making this decision correctly will lead to the most long-run profit for a skilled player, players often put considerable study into what the appropriate starting hand standards are for the game being played.
Optimal starting hand standards can be very sensitive to factors such as the betting structure of a game, position, and the character of the other players, as well as the rules of the game being played.