Does a power supply draw its rated watts all the time?

I'm really confused when it comes to power supplies. I'm looking at a 650 Watt, bronze rated power supply.

Does this mean that it is constantly drawing 650 Watts? Or is that how many it pulls in an hour?

I currently have a 500 Watt Antec Earthwatts PSU, and I don't overclock much and have no need for heavy gaming, but it comes in a combo deal.

Am I throwing away my money on electricity, or does a power supply only draw what is needed - what your system is drawing - and not the full 650 Watts all the time?

That's its rated output. It will not output more than the load requires. Its input current demand will vary based on the load on its output. So, no, it won't be putting out 650 watts all the time.