Connection Oriented vs. Connectionless
The terms connection oriented and connectionless are descriptive words used to describe different kinds of communication.
Connection Oriented
Connection-Oriented means that when devices communicate, they perform handshaking to set up an end-to-end connection. The handshaking process may be as simple as syncrhonization such as in the transport layer protocol TCP, or as complex as negotiating communications parameters as with a modem.
Connection-Oriented systems can only work in bi-directional communications environments. To negotiate a connection, both sides must be able to communicate with each other. This will not work in a unidirectional environment.
Connectionless
Connectionless means that no effort is made to set up a dedicated end-to-end connection.
Connectionless communication is usually achieved by transmitting information in one direction, from source to destination without checking to see if the destination is still there, or if it is prepared to receive the information. When there is little interferance, and plenty of speed available, these systems work fine. In environments where there is difficulty transmitting to the destination, information may have to be re-transmitted several times before the complete message is received.
Walkie-talkies, or Citizens Band radios are a good examples of connectionless communication. You speak into the mike, and the radio transmitter sends out your signal. If the person receiving you doesn't understand you, there's nothing his radio can do to correct things, the receiver must send you a message back to repeat your last message.
IP, UDP, ICMP, DNS, TFTP and SNMP are examples of connectionless protocols in use on the Internet.