Developers use different Salesforce APIs to integrate with Salesforce orgs and access data. The main APIs include the SOAP API, which is the oldest and most commonly used for manipulating records and running queries, the Metadata API which allows modifying metadata like layouts and classes, and REST APIs which provide alternatives to SOAP for various use cases like the Bulk API and Chatter API.

1. Developers use APIs to integrate with Salesforce Org
Which API and when to use
Purpose : Accessing data from Force.com or Database.com
Types of APIs :
1)SOAP API
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Most commonly used API
SOAP as a wrapper for API operations
This is the API that’s been around the longest
To create SOAP messages
*concatenate strings to build a SOAP envelope, it’s pretty laborious:
So using Tools to import the WSDL:
Visual Studio
Eclipse
Also use Toolkits : Java toolkit --- WSC
WSC:
1) Import .jar file
2) Gives a nice programming interface to create records, run queries, etc
3) SOAP API is oriented around accessing data and manipulating records
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2)Metadata API (another SOAP-based API)
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Gives an API to manipulate layouts, Visualforce pages, Apex triggers and classes
Querying information about your organization
updating or creating objects
full create, read, update and delete with the metadata API
Need a library on the client side or can use the tooling to import the WSDL
Force.com IDE use the Metadata API to make changes in an org.
3)Apex API
4)Apex SOAP
5)REST
6)REST API
7)Bulk API
8)Apex REST
9)Streaming API
10)Chatter API