A collaboration is a static construct that models the way a specific set of instances interact with each other. The instances communicate with each by exchanging stimuli in an overall effort to accomplish some purpose.
An extension point is a feature of a use case and acts as a reference to a location in the (extended) use case at which the behavior of the use case may be extended by the insertion of additional behavior from another (extending) use case. A use case may have any number of extension points and these are used in conjunction with the extend relationship which contains reference to the extension points and conditions which govern whether the extending behavior will be inserted.
A use case represents the behavior of the entity, describing the interaction between the actors and the use case in a series of actions, with possible variants. It describes the value the entity provides to the actor with which it communicates. The use case never describes "how" the value is achieved, just "what" the value is.
An use case instance is a specific instance of a use case. It represents the behavior of the subject, describing a particular instance of the interaction between the actors and the use case. A use case instance communicates with one or more instances of actors sending message instances to achieve the value represented by the use case instance. The use case instance never describes "how" the value is achieved, just "what" the value is.