Unpaid leaves is any time-off taken by an employee, which is not paid for by the employer. Most of the time, these leaves are requested by the employee for personal emergencies or situations when the leave duration exceeds the eligible limit of paid-leaves he/she is allowed in the organization. However, at times this time off might be requested by the Employer itself too. 

Unpaid leaves are generally not classified based on "reasons" (unlike Paid leaves). 

For the sake understanding, below we've outlined a couple of commonly used terms for Unpaid Leaves:

Leave Without Pay (LWOP / LWP): 
This is generally a temporary time-off period requested by the employee, knowing that it will not be paid for by the employer. Most times, this leave is granted by the supervisor/manager/head of departments when the circumstances compel the employee to take the leave. And the reason it becomes a "Leave Without Pay" is mostly because the leave is not falling within the policy limits. For instance, the employee may be eligible for only 2 leaves, but he's requires more than 2 days leave. If the manager/management agrees to this leave, they may approve it as "LWOP/LWP".

Loss of Pay (LOP): 
This particular term is associated with leaves taken by the employee, without prior permission or in spite of exceeding his/her eligible quota of leaves. As a matter of policy, companies respond to these situations by treating that period of absence by deducting a pro-rated amount from the employee's salary.