Mostly, in every job where the person is supposed to be taken as an employee of the company (not as a contractual associate), that person is put on probation for the initial few months. This is called the Probation Period in a job.
In this period of time, generally, the abilities to work for the given job role and the overall character of that person at work place is tested in order to finally evaluate whether or not he is the right candidate for that job and for the organization at large.
During the probation period, the work of the candidate is closely monitored and the progress is deeply assessed. So, the person should be extra careful in completing the work diligently and adhering to the timeliness, good conduct at work, showing dedication, creativity, etc.
Once the probation period is over, the employer evaluates the progress and decides whether to continue your service or terminate it. If they so decide to continue to take your service, then they would at first release a confirmation letter to you and then provide the appointment letter as a payroll employee of the organization.