In software development and product management, a user story is one or more sentences in the everyday or business language of the end user or user of a system that captures what a user does or needs to do as part of his or her job function. User stories are used with agile software development methodologies as the basis for defining the functions a business system must provide, and to facilitate requirements management. It captures the 'who', 'what' and 'why' of a requirement in a simple, concise way, often limited in detail by what can be hand-written on a small paper notecard.
http://en.wikipedia.org/wiki/User_story
The product backlog is an ordered list of requirements that is maintained for a product. It consists of features, bug fixes, non-functional requirements, etc.—whatever needs to be done in order to successfully deliver a viable product.
http://en.wikipedia.org/wiki/Scrum_(development)#Product_backlog
A sprint (or iteration) is the basic unit of development in Scrum. The sprint is a "timeboxed" effort; that is, it is restricted to a specific duration. The duration is fixed in advance for each sprint and is normally between one week and one month, although two weeks is typical.
http://en.wikipedia.org/wiki/Scrum_(development)#Sprint
At the beginning of every sprint cycle, a sprint planning meeting is held. Each day during the sprint, a project team communication meeting occurs. And the end of a sprint, a Sprint Review Meeting is held to review and demo the work.
http://en.wikipedia.org/wiki/Scrum_(development)#Meetings
Timeboxing is used as a project planning technique. The schedule is divided into a number of separate time periods (timeboxes), with each part having its own deliverables, deadline and budget.
http://en.wikipedia.org/wiki/Timeboxing
Velocity is a capacity planning tool sometimes used in Agile software development. Velocity tracking is the act of measuring said velocity. The velocity is calculated by counting the number of units of work completed in a certain interval, the length of which is determined at the start of the project.