As a college or university student, you probably did not think too deeply about the job titles of the people teaching you. Everyone seemed to be involved in both research and teaching, making the terms 'lecturer' and 'professor' apparently synonymous. However, if you are considering a career in academia, especially if you wish to work abroad at some point, it is worth knowing the precise differences between these two roles.
Professors are expected to conduct academic research in their areas of specialization. Their research outputs are measured by the number of scholarly articles they produce and the quality and reputation of the journals in which they are published. Professors also write books and may be awarded grants to pursue particular avenues of research. A senior lectureship, however, focuses entirely on teaching; there is no research requirement.
As well as conducting research, part of a professor's job is to teach. Professors, including assistant and associate professors, will give lectures in their areas of expertise. More senior professors will supervise doctoral students and people in postdoctoral positions. A small number of people are awarded research professorships, which do not involve any teaching duties. A lectureship is a teaching position only, with a focus on lecturing. As lecturers do not have any research responsibilities they are not usually asked to supervise doctoral students.
The academic career path in the U.S. is directed towards achieving a tenured professorship. Tenure effectively means that you have a job for life, barring instances of unprofessional conduct. It was introduced to protect academics who were working in potentially controversial areas of research. Having served in postdoctoral roles as as an assistant professor, most academics would expect to achieve a tenured associate professorship up to ten years after completing a PhD. A lectureship, on the other hand, is not a tenured role. Although there are plenty of teaching positions available, a senior lectureship is the most senior job available in that field.
In Europe, the title of professor is given only to the most senior academics. Professors are heads of department or in charge of specialist research centers. Professorships are also awarded to people who hold a named chair in a particular area of expertise, such as the Coulson Professor of Theoretical Chemistry at the University of Oxford. Other academics tend to be defined as researchers or lecturers. Someone who might expect to be appointed as an associate professor or professor in the U.S. could find that the equivalent position abroad is that of senior lecturer.