Bridge pattern is categorized as a structural pattern in GoF because it structurally decouples abstraction from its implementation so that the two can vary independently. However, it is very similar to Strategy pattern which is categorized as a behavioral pattern.
Instead of having an abstract class defines the interface to the abstraction and its concrete subclasses implement it in different ways all in one class hierarchy, bridge pattern separates implementation from domain abstraction so they can be reused or extended independently with respect to its own inheritance hierarchy.
Abstraction encapsulates (has a reference to) Implementation and can initialize Implementation class via constructor of abstraction. As a result, you may have any combination of different Abstraction and Implementation. For example, image format like JPG or PMG is a kind of Abstraction class, where displaying on Widows or Mac is a kind of Implementation. The association from Abstraction to Implementation is a 'basic aggregation' (as opposed to composition aggregation) which means the child class (Implementation) instances can outlive its parent class (Abstration).
Abstraction encapsulates (has a reference to) Implementation and initialize Implementation class via constructor. As a result, you may have any combination of different Abstraction and Implementation.
when to use bridge pattern
Bridge vs. Strategy (very similar)