Containers are data types which can store a collection of objects, mostly homogeneous in nature.
C++ standard contains containers library that contains class types for different types of containers. Containers library typically contains class templates for these containers which can be used as containers for different data types. Examples of class types for these containers can be std::vector, std::array, etc.
Info
Container classes mentioned above are not fully container classes as per C++ standard. C++ standards provides a list of requirements that need to be implemented by a class type to be a container. These class types implement some part of those requirements.