The dual tuners enable both standard-definition (SD) and high-def (HD) programs to be watched and recorded. It's up to you to supply the antenna. Alternately, you can plug in a raw cable TV connection (straight from the wall jack) to get unencrypted "ClearQAM" TV channels. Just be aware that more and more cable systems are starting to lock out these channels, so selection may be increasingly limited.
The box itself is quite small--about the size of a pack of cigarettes--with just a power light at the front and an Ethernet port and RF cable connector for the antenna on the back.
The HDHomeRun has no wireless capability, so it needs to be put someplace that has access both to an Ethernet cable (for your home network) and an antenna (or cable TV connection), as well as an AC power source. That said, it doesn't have to be near your TV or computer at all--it could be set up in a spare bedroom, an attic, or any place else, so long it was access to those three key connections.
Setup on a Windows machine involves downloading a small app from SiliconDust called HDHomeRun QuickTV. Once installed, you use Windows Media Center to actually watch and record TV shows. (The HDHomeRun is also compatible with third-party DVR software, including BeyondTV, SageTV, and TotalMedia.)
Running the drivers on a PC, the program detected the HomeRun instantly and downloaded the latest firmware. Heading to Windows Media Center, the Live TV option found the tuners without any extra fiddling and let me record one program while watching another. If you have two computers using the tuners, the device will allocate one per PC.
When running it on a wireless network, I found that the HDHomeRun required a lot of bandwidth; even with with a 20Mbit Wi-Fi connection the feed was unwatchable on my PC. I could only get a stable picture by connecting an Ethernet cable. The picture was quite good, though, with no noticeable artifacts. Your wireless mileage may vary, but if you live in an apartment with other routers around, you may find you have no option but to go wired.
If you're interested in Mac performance, please see the Elgato HDHomeRun review.
If you have a solid signal to your router, then the HDHomeRun could be a good option for PC users, with a high-quality picture and relative ease of use. We'd still recommend wired if you want to stream HD or more than one channel.