In British universities the ranking is usually:
Lecturer, roughly equivalent to 'Assistant Professor' in American Universities.
Senior Lecturer, somewhere between the American Assistant and Associate Professor.
Reader, roughly equivalent to the American Associate Professor
Professor, the American Full Professor.
All of these may give lectures, conduct seminars, and assist students with Master's and PhD work. The difference is mainly in the administrative reponsibilities they have within the department, and the amount of time they may have for research.