While Murgatroid and Alex P are right that a complete understanding of Layers would require a full quotation of the recommended references, there is a space here for a broad overview.
Fundamental Concepts of Layers
Magic has a lot of effects that modify permanents in play. Often times these effects can be confusing or mutually exclusive when stacked, so the rules for how to deal with modification effects are very detailed.
The basic concept is called the Layer System. At any point in the game when you have to understand what kind of object a permanent is you start with the base object and then apply all existing effects that are modifying (or could modify it) one at a time until they have all been applied.
The order in which these effects are applied is largely determined by the type of modification being done. For example, Type changing effects (like turning a land into a creature) are applied before rules changing effects (like giving a creature vigilance) which are applied before power/toughness adjusting effects (like giving a creature +3/+3). The categories of modification are referred to as "layers", and there are 7 layers with several subcategories.
Inside a layer or sublayer, order is determined first by dependencies (an effect that makes all lands creatures will be applied before an effect that makes all creatures goblins), and then finally by "timestamp", the time when the effect came into existence, with older effects being applied first. Newer effects can and will overwrite older effects if there is a conflict.
There are no distinctions between effects that are being granted temporarily ("until end of turn") by oneshot effects and effects that are being granted statically by a permanent on the battlefield. All that matters is the type of effect and the time the effect began applying.
The result of all this is a system that about 99% of the time works in the most intuitive fashion possible, and mostly only fails to be intuitive when there is no single intuitive answer (I'm looking at you, power/toughness swapping). That's honestly pretty impressive considering how many bizarre interactions it has to handle (Opalescencex2/Humility being the most famous).