When a computing process wants to keep informed about new information, one technique engineers use is a polling mechanism. Polling is when you go check in a known location to see if there's anything new waiting for you there. Engineers debate about setting polling intervals and when not to use a polling mechanism, and so on, because polling consumes resources. You have to be careful when designing a system to make sure it doesn't demand that a computer do too much polling.
But nobody seems to mind when you design a system that demands that the user do lots of polling. Your mailbox at home, your work inbox, your email inbox, your other email inbox, your other other email inbox, your Twitter page, your LiveJournal inbox, your facebook inbox, your voicemail, your cell voicemail, your RSS reader... all of these demand that you go check them periodically. Which is really not an efficient use of your processor. And every time somebody adds a cool new communication service to the world, they add one more place that everybody has to poll.