Moving files between your iPad and your computer isn't hard, and with apps such as iWork (Keynote, Pages, and Numbers), it makes your life very easy. When you add in iWork.com everything's available when, where, and how you need it.
There are two keys to sharing files in this way. The first is that you share files with a specific app on your iPad. (Each app has its own area for documents.) Not all apps support file sharing (it doesn't make sense for many apps). But when it comes to iWork, there's no question that file sharing is critical, and all three apps support it.
The next key is iTunes: that's what actually manages the sharing.
With your iPad connected to your computer with the USB cable (or via a dock), launch iTunes. Go to the Apps tab as shown here:

Scroll down to the bottom of the window to get to file sharing. Select the app you want to share with at the left. (If your app isn't listed, it doesn't support file sharing.)

At the right you'll see the files on the iPad the app is willing to share. In the case of the iWork apps, these are files that you have exported. You choose the document from your Document Library using the Export command as you see here. Other apps may use a different interface.

Once you've exported the document, it will show up at the right side of the File Sharing section of the Apps tab in iTunes. Select it and then click Save To at the bottom to save it to any location you want on your computer.
The other direction is just as simple. Save the document somewhere on your computer (or on a network location to which you can connect). Go into the File Sharing section of the Apps tab in iTunes and click the Add button in the lower right. You'll be prompted to locate a file on your computer or network; when you do so, it will show up in the list and be downloaded to your iPad.
The only trick is that your iPad needs to be connected with a cable to your iTunes-running computer. (It doesn't show up in iTunes unless it's connected, so that means you can't get to the Apps tab.) It's really not a hassle. After you do it once or twice, you have it down pat.