A task is called process under Unix. A process is an autonomous activity that executes a program: it is a dynamic entity that is born, lives independently, or communicates with other processes, which in turn creates other processes and dies.
When a user starts a Unix session, the system creates a process that executes a shell (by default). This process will live until the end of the session (disconnection). At each command of the user, this process (parent) will create a process (child) to execute the command in question.
A process is characterized by a number of information stored in a kernel table: a unique identifier (PID), the identifier of the parent process that created this process (PPID), the process owner (UID), the owner group (GID).