Sensei means teacher and is used as respectful form of address for instructors and karateka of very high rank. They generally sit on the joseki (Shinzen or shomen) side. When entering a dojo bow to the joseki (Shinzen or shomen) first and then to sensei then ask for permission to train in his or her dojo. Whatever he/she says overrules any other prescribed regulations. It is for this reason that when visiting another dojo it is courteous and part of customary etiquette to follow the directions of the sensei regardless of what you have been told somewhere else. This does not infringe upon your rights and beliefs, indeed you are considered as one of his/her guests.
Sempai generally indicates the senior member who sits first in the line opposite to the teacher. You should always be listening for what he/she says as he often shouts out most of the commands. In reality sempai means “senior/elder” and indicates anybody with more experience than you. You will be kohai(junior) to some people and sempai to others. This apparently rigid hierarchy is often misunderstood and some people take advantage of it to dominate others. These individuals show very little understanding of karate and command very little respect from the karate community. To be sempai does not mean you command automatic respect, but places great responsibilities upon you and requires patience, maturity, knowledge and discipline. Indeed the sempai is a leading example, he is required to show the basics and it is his duty to take care of the kohai and their safety. Therefore, if you feel you can learn from somebody sit further away from the joseki (Shinzen or shomen) than him or her as a pure form of respect, they will not fail to notice and soon they will pay particular attention to you. The sooner you start to respect others the greater will be your achievements.
As already mentioned kohai is anybody with less experience than you, literally it means ” junior” . Please help anybody who seeks your attention, not with the manner of someone who knows more and arrogantly lectures, but as an example, with the simplicity and modesty of one who is passing on some knowledge he acquired through real work, hardship and strife. Remember, however high in grade you will be one day, there will always be someone higher than you. It shows good disposition and attitude to behave as a learner on any occasion.