What is the difference between mAh for external battery packs for iphone?

I want to buy an external battery pack for my iPhone 4 and I see that there are many different levels to choose from. I see packs with 1000mAh, 1800mAh, 1900mAh and so on up to 5000+mAh. My question is what difference does the mAh make in a battery pack? Is it the amount of charge it gives the phone? Is it the length of time it takes to charge? I want to be more educated before I make a purchase.