An interline flight is an agreement between airlines to coordinate passengers with an itinerary that uses multiple airlines, without having to check in again or deal with their baggage at the stopover.
Codeshare agreements is where airlines operate flights on behalf of another airline, using their flight code. For example, a ticket you purchased from Finnair but is operated on a British Airways plane, by British Airways. Codeshares are beneficial as it allows customers
to reach new destinations that are not served by their own aircraft expanding destinations without actually flying there, and focus their services on their own destinations.