Premium rounds are very misunderstood. A premium round is usually an armorer-piercing composite rigid (APCR) or high explosive, anti-tank (HEAT) round. They cost significantly more credits than a normal round, often 50x more expensive. They can also be bought with gold, the cash-bought currency in the game. It used to be that you could only by premium rounds with gold, hence people call them 'gold rounds.'
Premium rounds have higher penetration values than standard AP rounds. Against tanks that you can penetrate with a standard AP round, they're usually of no benefit. Against higher tier tanks with more armor, though, they can often give you a fighting chance. Since they're very expensive, they should be used when needed, not all the time. Many people though refuse to use them or complain about them because 'you don't have to aim' when using the rounds. They feel premium rounds put the game into 'easy mode.'
What they're missing, though, is that winning makes you much more cash than losing. And winning a game with judicious use of premium rounds is much more profitable than losing without using any. They're no different than taking a repair kit or fire extinguisher. They cost money to use, but when used at the right time will totally save your ass. I use premium rounds quite a bit, but I consider them just another tool.
What sucks, though, is that a number of tanks more-or-less NEED premium rounds to be palatable. The biggest offenders here are the French and American auto-loaders. With 30 to 50 seconds needed to reload a magazine, it's not practical to switch to premium rounds when you need them. So often times you'll load premium just so you have them ready. If the first tank you run into is a Pz38nA then you wasted your money. However if the first tank you run into is a E100, you're OK. Other tanks (tier 7 mediums come to mind) simply have terrible penetration with standard rounds. If flanking is not an option and you need to take frontal shots against T8 and T9 heavies in a T7 medium, you need premium.
I usually take about 75% standard, 25% premium and one or two HE shells. HE shells are more or less worthless because they do terrible damage. But they're more likely to cause a little damage even if they don't pen. So if you need to hit an enemy on cap to reset, HE can be useful. But that's about it.