Junior faculty are hired as assistant professors for a four-year term. After which they are usually renewed for an additional three-year term. At the of that second term either they are promoted to associate professor with tenure or their contract is not renewed and they need to find another position.
An assistant professor is hired based on potential and promoted to tenure based on accomplishment.
It is rare to not renew a candidate after the first term, happening only if the department feels there is little chance that the faculty member will received tenure after second term.
Since tenure requires a long-term commitment from the university, the department, the dean and the university put considerable effort in vetting the case. The candidate first puts together a tenure packet, with CV, detailed research and teaching statements, a collection of publications and list of potential letter writers. The department sends the packets to senior people in the field both on and off the list given by the candidate. Ten or more review letters are not uncommon for a tenure case. The tenure case works its way through the system from the senior members of the department through the dean, provost and so on. Many universities have a tenure committee that reviews all cases for the provost or president.
The final decision is based on several parameters including the letters, publications, teaching, grants, service to the university and academic community and how well the faculty member fits in the department. The weights given to each item as well as how high the tenure bar is held differs greatly between universities. You can get a good feeling by how recent tenure cases went in the department.
Can one come up for early tenure? Can one get credit for years as a postdoc, research scientist or an assistant professor elsewhere? Or can one "stop the tenure clock" for illness, a new child or other leaves of absences? Can one be promoted to associate professor without immediate tenure if needed? Can one get an extra year to search for a new job if not promoted? Will the candidate have access to the review letters? If the answers to these or other questions concern you, best to bring them up before you accept the job.