In terms of what many vendors call unlimited service, Web Hosting Pad's terms of service indicate that its definition of unlimited is what it calls "incremental." Basically, as you need more capability, it wants to discuss that with you, both to help you get the most out of its services, and to make sure you're using its systems without abusing them.
When it comes to server operating systems, Linux is typically the default option. Still, some services offer a choice of Linux or Windows hosting. If you have specific server-side applications that require Windows, such as SQL Server or a custom application written in .NET, then you need to make sure your web host has Windows hosting. But don't let the idea of a Linux host intimidate you. Nowadays, most web hosts offer a graphical interface or a control panel to simplify server administration and website management. Instead of typing at the command line, you'll click easily identifiable icons.