For just about every installation of Salesforce, a Sandbox will be used. What is a Sandbox? It is a copy of your organization in a separate environment that can be used for a variety of purposes (such as testing and training) without compromising the data and applications in your Salesforce.com production organization. However, the Sandbox is not a one-size-fits all sort of deal. In fact, there are 4 different Sandbox types and each of them have different levels of functions and features.
A Developer Sandbox is the simplest and smallest Sandbox. Use this Sandbox for development and testing in an isolated environment.
Developer Pro Sandbox
A Developer Pro Sandbox can handle larger data-sets than the Regular Developer Box. Use this one for more development and quality assurance tasks as well as integration testing or user training.
Partial Copy Sandbox
A Partial Copy Sandbox is primarily a testing environment. This Sandbox will carry over some records from your production environment, but not all. Tasks such as user acceptance, integration testing, and quality assurance are the best uses here.
A Full Sandbox gives you everything. Full sandboxes are a complete replica of your production org, including all data, such as object records and attachments, and metadata. Only Full sandboxes support performance testing, load testing, and staging. As the largest Sandbox, this one is not recommended for development, but rather full scale testing and QA.
These 4 different Sandbox types, when understood and used according to their strengths, give organizations the flexibility to develop and customize with ease.
-Ryan and the Salesforce Guys