Japanese has (at least) two words for "teacher".
The word "sensei" is pretty well-known even among people who don't speak Japanese, but did you know that you shouldn't use sensei about yourself?
Here's what the textbook has to say:
"Use 'kyōshi' for yourself and the respectful 'sensei' for another person."That's a pretty good starting point. But there's a bit more to it than that.
1. Kyōshi = school teacher
Kyōshi means the academic kind of teacher, someone who teaches in a school:
(watashi wa) kōkō no kyōshi desu.
I'm a high school teacher.
2. Sensei is a title
Sensei, however, is a respectful title, and should be used when talking about other people:
彼は中学校の先生です。Watashi wa sensei desu is best avoided.
kare wa chūgakkō no sensei desu.
He's a junior high school teacher.
3. Sensei = master
Sensei can also be used more generally for a person who teaches something.
People who teach flower arranging or martial arts, for example, are sensei:
ohana no sensei
karate no sensei
茶道の先生If you're talking about yourself, however, you still shouldn't go around calling yourself sensei. You can use the verb 教える oshieru (to teach) instead:
sadō no sensei
teacher of tea ceremony
（私は）お花を教えてます。Certain types of professionals such as doctors or lawyers are also sensei (but again, not kyōshi).
(watashi wa) ohana wo oshiete imasu.
I teach flower arranging.
Sensei is attached after teachers' names instead of san:
山本先生It's pretty common to drop the name, too, and just call your teacher sensei:
= Mr/Ms Yamamoto; “Yamamoto teacher”
Sensei, ohayō gozaimasu!
“Good morning, teacher”
But you can’t call your teacher Yamamoto Kyōshi or greet them with “kyōshi, ohayō gozaimasu”... unless you want them to think you're a bit strange.
Related: Hey! What's That お Doing There?