What are the various methods for integrating with Salesforce?

Several Salesforce integration methods:
User interface integration is the process of combining the UIs of two or more apps to produce composite apps with little or no modification on the UIs of the separate apps.

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Apex Web Services is used for inbound logic, and Apex Callouts is utilised for outbound logic. It often handles scenarios in which business logic is distributed among several apps in order to accomplish the entire end-to-end business process.