--> The outbound connection is the connection that is originated from the networks behind higher security level towards to the networks behind lower security level.
--> The inbound connection is the connection that is originated from the networks behind lower security level towards to the networks behind higher security level.
--> By default TCP/UDP connections are allowed for outbound traffic.
--> The Outbound connection is automatically being inspected and does not require access control list for traffic being returned ( Traffic which is going from LAN to Internet).
--> The Inbound connection is unsecure and requires access control list for allowing the connection( Traffic is coming from Internet to LAN).
--> Each and every interface on ASA requires basically IP address, Interface name and Security level basically.
--> By default the security level is setup automatically, if we put interface name inside or outside.
--> For Inside the security level is 100 and for outside the security level is 0 ( Other interfaces we have to setup security levels manually by using command).
Reference : Narbik Workbooks