What are different APIs in salesforce.com?

Hi Suraj

Different APIs in salesforce.com are:

REST API - Access objects in your organization using REST.
SOAP API - Integrate your organization’s data with other applications using SOAP.
Tooling API - Build custom development tools for Force.com applications. Coming soon!
Chatter REST API - Access Chatter feeds and social data such as users, groups, followers, and files using REST.
Bulk API - Load or delete large numbers of records.
Metadata API - Manage customizations in your org and build tools that manage the metadata model (not the data, itself).
Streaming API - Provide a stream of data reflecting data changes in your organization.
Apex REST API - Build your own REST API in Apex. This API exposes Apex classes as RESTful Web services.
Apex SOAP API - Create custom SOAP Web services in Apex. This API exposes Apex classes as SOAP Web services.
Data.com API - Data.com provides 100% complete, high quality data, updated in real-time in the cloud, and with comprehensive coverage worldwide.

Hope it may helps