What does the special character “!” mean in PowerShell?

Question:

What does the special character ! mean in PowerShell?

Or a site which lists all special characters and meaning.
Example:

$string = blah
!$String

$string = blah

!$String

(Returns $false)

PowerShell uses the ! character as an alias for the logical -not operator:

$true
!$true
$false
!$false

True
False
False
True

$true

!$true

$false

!$false

True

False

True