Nice creamy ice cream has tiny tiny little ice crystals in. They're so small they feel smooth and they all melt uniformly. Cheap ice cream has bigger ice crystals in it, so it tastes a little gritty and it doesn't melt evenly. There's a few ways to make really nice creamy ice cream:

• Get it cold, very quickly - To some extent everyone tries to do this, they get the mix almost freezing, and then churn it at very cold temps. And actually "slow churned" ice cream is churned very quickly, just in a different way that freezes it faster
• Have a lot of fat and/or sugar - little bits of fat get in between the ice crystals keeping them from joining together in to big ice crystals. And sugar will hold on to water to slow down the freezing process, which keeps everything more uniform
• Put in stabilizers - things like guar gum or carrageen. These form gels instead of freezing, they make the ice cream smooth, but they also make it taste and feel artificial, even waxy for some very cheap ice creams
• Don't store it very long - even after it's churned the crystals in ice cream will continue to grow. This is why ice cream that's been in the freezer for a long time tastes gritty. Or if it's been thawed and refrozen this will happen too.
• Don't store it at all - this is basically what they do with gelato. It actually has a very low amount of fat and sugar, and it comes out super creamy, but it won't stay that way. All real gelatos will get gritty very quickly. This is why most good gelato shops make their gelatos on site everyday. And why the stuff in groccery store freezer section called gelato is usually just very rich ice cream.