A status effect in the abstract is a persistent consequence of a certain in-game event or action, and as such innumerable variants exist across the gaming field.
Players may acquire status effects by consuming items, casting spells on themselves or each other, activating devices in the world, interacting with NPCs, or remaining in a particular location.
In addition, many games have weapons, armor, or other equipment that can mitigate status effects or prevent a character from getting one in the first place.
Depending on the game, some increase the chance to escape suffering the effect each time the player may potentially receive it, while others grant complete immunity.
Buff is the term generically used to describe a positive status effect that affects mainly player or enemy statistics (usually cast as a spell).