Agile estimation, from planning poker to bucket sizing

Agile Project Management is an iterative and incremental approach to software development projects supported by a cross-functional team. It allows for flexibility and frequent delivery of product iterations, and enables teams to respond to changes quickly. Agile follows a framework, with detailed project planning and estimation for each sprint before execution. Accurate estimation is important for planning and managing the effort to deliver a new feature or product. 
This extensive article covers four methods for estimating work: Planning Poker, Asynchronous, Relative Estimation, and Bucket Sizing, with explanations on how to use each approach.