Used in Derry as a greeting that goes beyond a simple hello. When you say "yes" to someone in Derry it really means "well my good friend, it's most excellent to see you and I hope you are doing well and achieve all you undertake."
It can be used to start a large and varied conversation, although has been known to be a conversation on it's own:
Person 1: "Yes!"
Person 2: "Yes."
*end of conversation*
"Yes" is very handy to use if you don't have time to talk to someone, for example if you are in a hurry to get somewhere but pass an old friend along the way. By exchanging "yes's" in a couple of seconds it is as effective as stopping for a cup to coffee and a chat about good times.
A word of warning, if you use "yes" as a greeting outside of Derry, you will generally be given funny looks...