Online charging system (OCS) is a system allowing a communications service provider to charge their customers, in real time, based on service usage.
Typical events are SMS, MMS, purchase of content (application, game, music, video on demand, etc.).
Similarly, if you send a text message it may cost you Rupee 1/- and that's it.
You subscribe to Caller Ring Back Tone (CRBT) which costs you Rs.30/- a month irrespective of the number of calls you receive in a month.
Utility services like electricity or water is charged based on overall usage, for a certain time duration.