Positive Time Management:
Positive Time Management is when an employee has to "Clock-in and Clock-out" each day. There must be a time entry or absence entry for every scheduled work day.
Negative Time Management is when an employee only has to record absences or "variations" to the work schedule - works overtime, works other than regular scheduled hours, vacation, jury duty, funeral leave, etc.
Hourly employees - those paid for each hour worked usually use positive time recording, Salaried employees, paid regular amounts regardless of time worked usually use negative time recording.
The details are as follows : There are 2 Mechanisms of time recording are available
1.Positive time recording :- Actual times are recorded
This method completely records all actual times, that is, all transactions such as actual working times, absences, and so on.
2.Negative time recording :- Deviations from a work schedule are recorded.

Recording Only Deviations to Work Schedules
This method records only employee time data that represents exceptions or deviations to the work schedule assigned to employees. Here you can include the most current data, such as employee illnesses, schedule and record substitutions, and enter annual leave for employees.
SAP provides various Info types for time recording purposes.
All above explanations are correct, however to simplify :
In Positive time, where you need to maintain all in time out time either through Time Recorder or some time it is uploaded after maintaining manually.
Example:
When you record in time as 8.30 am and out time as 19.30 pm. against your DWS 8.30 am to 16.30 pm. applicable on that day, the additional 3 hrs automatically gets into Overtime...whereas if you don't record your in time or out time any day against your X planned working hours in DWS, it means its your absent that day.
Whereas in negative time: You maintain these additional 3 hrs in Overtime Infotype. and absence in 2001 infotype for that PERNR.