There are two main types of hammock stands. If you already own a hammock, it'll be pretty obvious which one you should go for. However, if you're still deciding, here's what you should consider:
Standard hammock stands are designed to hold a traditional hammock horizontally, and are just what you need if you want to use it to lie down, relax or take a nap in the garden;
Hammock chair stands are taller models designed to hold hammock swing chairs vertically, which makes them ideal to chill out while reading a book or to work on your laptop rather than to lie down.