What is the difference between inbound and outbound webservice?

Explanation#1:

Inbound / Call-In: - Receiving Request

Outbound / Call-Out: - Sending Response

Mid Server - Acts as interface between SN and a outer network

Explanation#2:

Inbound means that other applications make calls to ServiceNow's REST API to get information from SN, or create/update records.

Outbound means that ServiceNow makes calls to other applications' REST API to get information from the other application or create/update records.

Explanation#3:

inbound is basically when some other tool hits your instance(wsdl) with a payload and you process it and acknowledge it. EG, if an incident gets created in a third party tool then an incident gets created in a service now . Here we accept the request and process it.

When we hit any other tool then its outbound. EG exactly opposite to the above eg, when an incident gets created in Service now, a similar incident should get created on the other tool. Here we prepare the payload and hit the other tool's wsdl.

You can get videos here:

Integration Best Practices - Inbound Web Service - YouTube

Integration Best Practices - Outbound Web Service - YouTube

Resource Link: https://community.servicenow.com/thread/178997