There are different type of integration available like inbound, outbound, scripted web services, data source. Integration can be uni-directional or bi-directional based upon the business use case.

There are many ways to achive it, using REST, SOAP, EMAIL Inbound etc.

You can refer few links for your information:

SOAP message

Create a REST message

Service-now.com Tutorial : SOAP Message Intro - YouTube

ServiceNow Integration (REST) - YouTube

Integration Best Practices - Inbound Web Service - YouTube

More info you can search here: Product Documentation | ServiceNow

What type of integration are you looking for?