I know this answer! Yes, they are digital and yes, there are a few ways to read them. Whether you download them or read them online (you can do both from how I understand it) depends on how you plan on reading them. You can read them on the website (via your account library) or any of the ComiXology apps for your Windows 8, iphone/ipad, Kindle Fire, Android device.
From ComiXology’s FAQ section: Where Can I Read These Comics?
When using your browser to read on our website, you will need an active internet connection. When using an app, you can read downloaded comics offline as long as you are logged in.
So, for me for example, I take that to mean that I can access these comics via my desktop computer browser through their website as long as I’m online. If I want to read them offline, I need to use one of the apps (like I am with my Kindle Fire) to actually download them to the device to read whenever I want, internet access or not.
Does that help?