Macs and Windows PCs have the ability to control other Macs and PCs via VNC and Microsoft's Remote Desktop Protocols. The Mac has this built-in using the Mac OS X Sharing features (VNC) and it's a quick and easy download to enable this on the PC as well. Desktop Connect allows you to use your iPad to control either a Mac or PC on your network or over the internet wirelessly. I use this functionality from time to time to check the status of a process running on a Mac in the other room or on another floor or even from across the world when I'm traveling. Perhaps it's a video render or a download/upload that requires further attention once it's done. 

With Desktop Connect I can not only see my Mac, but I can control it too

I've used other VNC compliant Apps on the iPad, but I must say that I'm very impressed with Desktop Connect's user interface. It quite frankly blows away everything else I've tried to date. The interface is full featured and easy on the eyes too. I also love the fact that you can setup and it remembers multiple computers to control. For example, I have the necessary ports open on my router and directed to my Mac mini Server to control it from anywhere I happen to be in the world. However, when I'm home I also from time to time control my MacBook Pro from afar. Just yesterday I was monitoring a clone backup from a distance directly on the iPad. I found the setup to be pretty straight forward and easy to get going. I haven't tried this to control a PC, but if the setup is anywhere near as easy as it was on the Mac you'll be up and running with an encrypted connection in no time. Another use case for this is to run technologies in your desktop's web browser that aren't available on the iPad like Adobe Flash or Java apps. Although I've got a screen shot above of Photoshop CS5 running on my MacBook Pro, let me level set expectations here. There is a small lag from the time you tap something until it refreshes your iPad display. This is normal for this type of connection. So don't run off after reading this thinking you'll be doing all your photo retouching, page layout and video editing from the beach on your iPad. Although I can run an App like Photoshop remotely, I would only use this to do some quick and basic things. I would NOT want to do this for any extended period of time. The lag would drive me insane and it's just not designed to turn your iPad into a "terminal".  Think of it more as you need to click a button, run a command, launch an App, look something up, drag and drop something from here to there. Perhaps you left a file at home that you need and you want to login and email it to yourself. Those are the kinds of quick things that this is well suited for. That being said, this is by far the best App I've seen in this category. I love the buttons at the top that give you quick access to an expanded keyboard with the keys you'd want such as CTRL, Command, Option/Alt, etc. as well as a mouse that you can tap to change from left click to right click, etc. 

You can get Desktop Connect here from the Desktop