A lecturer is someone who stands up in front of a class and gives an organized talk designed to teach you something. There are lots of lecturers at colleges and universities.
Although many professors lecture, in this country the title lecturer usually refers to a teacher who is not a permanent member of the faculty but one of the many college instructors who does not have tenure. The lecturer may even be hired for a single semester or year. The title is used slightly differently in England. There are some professional lecturers — people, like former President Bill Clinton, who make their living by going around the world giving speeches.