Despite the label, the assistant professor is not an assistant. He or she is an entry-level professor with all of the duties and responsibilities of other professors. Typically an assistant professor is hired right after finishing graduate school or after completing a postdoctoral fellowship (post doc).
The assistant professor participates in three sets of duties that come with a career in academia: teaching, research, and service. All professors do more than teach classes; they also must conduct scholarly work, specifically, present and publish their work at conferences and in peer reviewed journals. It is the professor's research that often is most critical in earning tenure although this varied by institution. The third set of duties that come with the job of being a professor is service. Service duties entail all of the administrative work that keeps a college or university running. Service includes sitting on committees ranging from creating and evaluating curricula to overseeing workplace safety. After about 6 to 8 years the assistant professor often is eligible for tenure and/or promotion to the next rank, associate professor.