The only definition of NoteProperty I've been able to find said they are properties inherited from the PowerShell environment. That doesn't really clarify what they are or how they are used. All the blog posts I've found that reference NoteProperty all seem to assume everyone already knows what they are. "NoteProperty" is even missing from the TechNet PowerShell Glossary.
What is a NoteProperty and how does that differ from a normal property?
NoteProperties are generic properties that are created by Powershell (as opposed to properties that are inherited from a specific dotnet object type).
Properties of PS custom objects will be NoteProperty, as will the properties of objects created with Import-CSV, or created by using Select-Object and specifying properties to select.