Network layers can offer two different types of service to the layers above them. They are connection oriented and connectionless. Connection oriented service is modeled after the telephone system. To talk to someone, we pick up the phone, dial the number, talk, and then we hang up. Similarly, to use a connection oriented network service, the service user first establishes a connection, uses the connection, and then releases the connection. The essential aspect of connection is that it acts like a tube. The sender pushes the objects (bits) at one end, and the receiver takes them out in the same order at the other end.
In contrast, connectionless service is modeled after the postal system. Each message (letter) carries the full destination address and each one is routed through the system independent of the others. Normally, when two messages are send to the same destination, the first one sent will be the first one to arrive. However, it is possible the first one sent can be delayed so that the second one arrives first. With a connection oriented service this is impossible. Each service can be characterized by a quality of service. Some services are reliable in the sense that they never lose data. Usually, a reliable service is implemented by having the receiver acknowledge the receipt of each message, so the sender is sure that is arrived. The acknowledgement process introduces overhead and delays, which one are often worth it but are sometimes undesirable.