how are these interfaces different?

you can refer to the below documentation as its very well explained

https://docs.fortinet.com/document/fortigate/6.0.0/handbook/349060/in-band-management

https://docs.fortinet.com/document/fortigate/6.0.0/handbook/234765/out-of-band-management

Biggest difference would be the OOB management requires dedicated interfaces for management meaning that you can not use them for any other purposes, while IB management gives you the opportunity to use an already in use interface in the configuration (it doesn't reserve an interface). 

For a more detailed explanation please refer to the documentation above.