There are following below difference between Offline Shipping Method and Online Shipping Method
The Online Shipping methods are those which actively consume the API of a 3rd party system owned by a shipping services provider in order to fetch the shipping cost and the information regarding the parcel’s delivery tracking
While
The Offline Shipping methods implements the shipping methods which do not involve a direct interaction with shipping carriers, so called offline shipping methods.
In case Magento 2, offline shipping methods as
Free Shipping
Flat Rate
Table Rates
Store Pickup