-тель is a fairly common suffix to form a noun that names the object performing an action of a verb. Here are a few examples:
- учитель (the only with the plural in -я)
- выключатель, переключатель
- родитель (quite unusual due to formation from a perfective verb)
It is not the only one, and you should know whether the word exists anyway. The one who buys is покупатель but the one who sells is продавец.
«Преподава́тель(ница)» is someone who teaches students in the university (it roughly overlaps with the English word 'professor'), «учи́тель(ница)» is someone who teaches people in school.
Преподавательница sounds a tad weird. (I know, dictionaries have this word but I guess you have to trust my word here. Or not :)). More generally, even though names of certain professions do have feminine forms, you need to exercise caution using them. Check the notes in the dictionary for разг./разговорное, прост./простонародное, уст./устаревшее and act accordingly.
The second point, it is possible to call a school teacher преподаватель. All it does, it makes your speech sound more formal. The opposite is not true. One cannot call a university professor учитель or учительница, these words are reserved exclusively for school teachers.