Now that Microsoft Office is out on the iPad. it just underlines what the iPad has become: a laptop replacement for many people. You don't need a keyboard for your iPad. But, if you're planning on an iPad being a place where you get a lot of writing done, having one can come in very handy. Yes, you can type on the iPad's screen instead, but a real keyboard frees up screen space, feels more comfortable, and isn't hard to set up via Bluetooth.
You can use a regular Bluetooth keyboard with your iPad, or go
with a combo case or accessory. I've played with a lot of keyboard accessories on the iPad: these are my favorites.
Before the iPad Air. most cases would fit the second-, third-, and fourth-gen iPad models equally. That's no longer the case. So, having used several new accessories and written on each, I've broken my recommendations down into categories of iPad: Air, previous generations, and iPad Mini. No matter who you are, there should be a good option.
Just keep in mind that none of these support trackpads, because iOS still doesn't allow for it. Sad, but true.