A leave is a period of time an employee is absent from his/her work. During this time, though the employee will be away from his/her primary job, he still continues his employment with the company, viz. he remains employed as a salaried employee with the company.
Mostly, a leave is expected to be requested in advance such that the supervisor/manager can arrange or plan for the team's work keeping in mind the potential absence of the employee who's going on leave.
However, there might be times when leaves are taken unplanned for unexpected circumstances or emergencies. At such times, the company policies governs whether the leave may or may not be allowed, and if allowed then how will it be treated with respect to the employee's leave eligibility and remuneration.
In situations when the organization is facing financial problems and is not able to bear the operating expenses arising out of wages/salaries every month, it may enforce a temporary leave on employees. These leaves generally don't include any remuneration but do continue the employment contract of the employee. This way once the financial problems are overcome, the company can re-deploy these human resources back on the job, and continue as usual.