Sandboxes create copies of salesforce organization in a separate environment. Without affecting your live instance, you can use them for development, testing and training. Before putting the changes in live production environment, Sandbox is a very important tool to test the changes to salesforce system. It is just like a way you wouldn’t build a house without drawing an architectural sketch first, before being deployed in the organization all developing and testing is done in a sandbox.

There are 4 kind of salesforce sandboxes that fulfil different need and support different activities:

Developer Sandbox: Developer sandboxes are intended for coding and testing by single developer, but more than one person can login to the sandbox. It’s free, as well it is not recommended for training as it can only accommodate 200 MB of data and may not be sufficient for training of large group of users.

Developer Pro Sandbox: Developer pro sandbox works same as developer sandbox but it can host large data sets than developer sandbox. You can use this environment for additional task such as data load, integration testing and user training. It is also free and gives you 1GB storage.

Partial Copy Sandbox: A partial copy sandboxes are intended visually for any development, testing, or training purpose. It gives you 5GB of data storage and 10,000 records per selected object. This type of sandbox is paid.

Full Sandbox: This is the only sandbox that support performance testing, load testing, and staging. It can provide an ideal environment for training, it is all the more necessary for its use in testing with accurate data.
Reasons to Use Your Salesforce Sandbox

Sandbox can be used for development, testing, and research without losing data in production environment. It is very important tool provided by salesforce.
It allows you to test your customization like workflows, validation rules, and data migrations. Most important thing, you can try out applications as well as configuration without compromising the application and your data in production environment. This way, nothing will change permanently before you know exactly what it does.
A Salesforce sandbox is an ideal testing ground for app development, since you can try new things without fear of breaking your app.