When to Use Salesforce Metadata API

Use Metadata API to retrieve, deploy, create, update, or delete customizations for your org. 
The most common use is to migrate changes from a sandbox or testing org to your production environment. 
Metadata API is intended for managing customizations and for building tools that can manage the metadata model, not the data itself.

The easiest way to access the functionality in Metadata API is to use the Force.com IDE or Ant Migration Tool. Both tools are built on top of Metadata API and use the standard Eclipse and Ant tools, respectively, to simplify working with Metadata API.

• Force.com IDE is built on the Eclipse platform, for programmers familiar with integrated development environments. Code, compile, test, and deploy from within the IDE.
• The Ant Migration Tool is ideal if you use a script or the command line for moving metadata between a local directory and a Salesforce org.

Read More: Which Salesforce API Do I Use in Integration?
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                   When To Use Salesforce Chatter API