So when I buy my OS in what form will it come (ex: CD ROM)
Legal versions are typically DVD disc(s).
What about this https://pcpartpicker.com/part/microsoft-os-gfc02050
In retail, they would give you a DVD Drive with a key code sticker on the enclosure. All you have to do is use that key code. This means that you do not need to use the CD disk that came with the key; you can install windows on your build through a usb disk drive, and activate windows with that key.
So this is a stupid question but... If I buy it off of the website is that considered "retail"
yeah, newegg/amazon are online "Retailers"
Not necessarily... There's retail AND oem that they sell... Just don't get the one labeled as oem and it should be retail. And that one you posted a link to is oem, not retail.
Check this retail or oem for Windows 7 Retail vs OEM.