APIs act as Web services interface for interacting with Salesforce.
REST API
REST API is mainly used for ease of integration and development. It supports smaller sets of data.
SOAP API
SOAP API cab be written in any language that supports Web services.
For example, you can use SOAP API to integrate Salesforce with your organization’s ERP and finance systems, deliver real-time sales and support information to company portals, and populate critical business systems with customer information.
Chatter REST API
Chatter REST API provides programmatic access to Chatter feeds and social data such as users, groups, followers, and files.
Bulk API
Bulk API is based on REST principles and is optimized for loading or deleting large sets of data.
Bulk API is designed to process data from a few thousand to millions of records.
The easiest way to use Bulk API is to enable it for processing records in Data Loader using CSV files. This avoids the need to write your own client application.
Use Metadata API
Use Metadata API to retrieve, deploy, create, update, or delete customizations for your organization. The most common use is to migrate changes from a sandbox or testing organization 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 Force.com Migration Tool.
Streaming API
Use Streaming API to receive notifications for changes to data that match a SOQL query that you define.
Streaming API is useful when you want notifications to be pushed from the server to the client. Consider Streaming API for applications that poll frequently. Applications that have constant polling action against the Salesforce infrastructure, consuming unnecessary API call and processing time, would benefit from this API which reduces the number of requests that return no data.Streaming API is also ideal for applications that require general notification of data changes. This enables you to reduce the number of API calls and improve performance.
Apex REST API
Use Apex REST API when you want to expose your Apex classes and methods so that external applications can access your code through REST architecture. Apex REST API supports both OAuth 2.0 and Session ID for authorization.
Apex SOAP API
Use Apex SOAP API when you want to expose your Apex methods as SOAP Web service APIs so that external applications can access your code through SOAP. Apex SOAP API supports both OAuth 2.0 and Session ID for authorization.
Tooling API
Use Tooling API when you want to manage and deploy working copies of Apex classes and triggers and Visualforce pages and components, set checkpoints or heap dump markers, execute anonymous Apex, and access logging and code coverage information.
Cheers!!!