While @TheCompWiz's answer is good, I'd like to add my own version based on my own personal experience.
When you think about automating Windows deployments (anything Vista/2008 and beyond), it helps to separate the actual installation bits from the delivery of the installation bits to the destination machine. A Windows 7 DVD is the standard delivery mechanism for the default set of Windows 7 installation bits. It contains a small WinPE image (boot.wim) that loads on boot which is then responsible for writing the actual OS image (install.wim) down to the hard drive. Windows Deployment Services is also just a delivery mechanism. It can host both the boot.wim and install.wim so that clients can do the installation directly over the network with no physical media necessary.
But delivery of the installation bits is only a small part of the story. Creating your custom "image" (though it's really more of a hybrid image/automated installation) is generally what takes the most time and what most people care about. Others have mentioned using Sysprep to create your custom image. But using it directly and manually editing things like unattend.xml is way more trouble than it's worth. At the end of the day, many of the free and paid solutions for customizing Windows deployments are just wrappers and pre-made scripts that all tie back to Sysprep.
My personal favorite free tool for making custom Windows deployments is the Microsoft Deployment Toolkit (MDT) which recently released version 2012. All it really is is a bunch of pre-made scripts and some GUIs that work together with the Windows Automated Installation Kit (WAIK) to help you build your automated installation. You then have a variety of choices on how to deliver that installation to your clients (WDS boot.wim, bootable CD/DVD, bootable USB key, etc).
Out of the box, it's really not that difficult to build a quick image with some drivers, patches, and applications. Where it shines though is once you dig into making your own custom scripts. The sky is really the limit here. In my previous job, we were using it to deploy a single Windows 7 x64 image to about 1000 machines from multiple hardware vendors including dual-boot MacBooks and iMacs. MDT is really one of my favorite Microsoft products of all time. And did I mention it's free?