What the main difference between them?
Is it worth buying Gold and even Platinum Certificate PSU's?
Will a Gold Certificate PSU serve me more years than a Bronze Certificate one?
A bronze rated PSU will waste around 18% when at 100% load.
Meaning if you have a 600Watt bronze rated PSU running at 100% load, about 108 watts will turn to heat.
If you have a 1200W PSU running at 50%, a bronze PSU will waste 15%.
50% of 1200 = 600. + 15% = 90. That means the PSU will draw in 600W, and an additional 90W that'll turn into heat.
+1 to what hunter said, regarding whether it's worth the purchase or not, sometimes you can't decide.
Like, most high powered PSU's with huge AMP's are gold rated, some silver. (1000+Watt area).
IF your system will run 24/7 you want a gold rated PSU atleast, to ensure minimal wattage goes to waste.
Otherwise, if you're using less than 600W get a 80+ cert one. If you're using more, get a bronze cert one.
Do not however buy a cheap PSU.