There are 3 main types depending on what you're after.
1. Portable
Small size, Usb powered although you will pay more for the same capacity if a non portable version. Power and data through the same cable.
2. Desktop / mains powered
Slightly larger. Separate mains power supply.
Can get a large capacity for relatively cheap.
Separate usb cable to connect to pc for data transfer.
3. Network attached
You can access your files from different devices over your home wireless network. Can also access files over internet if setup properly.
Takes a bit of knowledge to setup and get the most out of it.
Size wise, get the most you can afford. These days 'normal' is around 500Gb (gigabyte). Although I would go for a 1TB (terabyte) if you can afford it.
A terabyte is a thousand gigabytes (roughly).
Some drives come with bells and whistles such as backup features but prob not needed.
Good brands are seagate or western digital.
Take a look at ebuyer for a good selection. ..
www.ebuyer.comHope this helps
Jas