A skill check is made when your character applies a skill to a task. This roll is made with a d20 (20-sided die), plus any skill ranks and ability modifiers. The skill check is successful if it equals or exceeds the difficulty class, or DC, of the task.
Difficulty class can either be the static property of an object, such as a trap, or it may be determined by the skill check result of another character. This last type of skill check is also called an opposed test, since you are opposing another character's skill check with your own. Several miscellaneous modifiers can also apply to a skill check, including racial abilities, feats, armor check penalties, and spell effects. In every case these modifiers are grouped together and added to a d20 roll, as shown below:
skill check = d20 + skill ranks + ability modifier + miscellaneous modifiers.
Certain skills allow for a take 20 where a fixed value of 20 is used instead of the d20 roll when the character is not in combat. Naturally, this means skill checks cannot be automatic successes if the die comes up 20 (and conversely, rolling a 1 is not an automatic failure), unlike attack rolls and saving throws.