New York City’s subway is an expansive network of interconnected trains. You can go a long way for a relatively low fare. And fairly quickly, too.
Using the rechargeable cards saves time and hassle. They’re bought from one of the machines where you can use a credit card (get the frequent flier miles). Then you swipe the card as you go through the turnstile. The display shows how much cash you have remaining on the card, so no surprises when to recharge.