Throw is used to test whether or not a block of code throws a terminating error. Throw has a number of different usage scenarios. The simplest is detecting whether a terminating error (of any kind) is thrown:

function Invoke-Something { throw } 
Describe Invoke-Something { 
    It 'Throws a terminating error' { 
{ Invoke-Something } | Should Throw 
    } 
} 

When testing for terminating errors, the subject of the test is placed in a script block (curly braces).

Pester allows testing of the error message:

function Invoke-Something { throw 'an error' } 
Describe Invoke-Something { 
    It 'Throws a terminating error' { 
{ Invoke-Something } | Should Throw 'an error' 
    } 
} 

Pester also allows testing of the fully qualified error ID:

function Invoke-Something { 
    $errorRecord = New-Object System.Management.Automation.ErrorRecord( 
        (New-Object Exception('an error')), 
        'AnErrorID', 
        'OperationStopped', 
        $null 
    ) 
    throw $errorRecord 
} 
Describe Invoke-Something { 
    It 'Throws a terminating error' { 
{ Invoke-Something } | Should -Throw -ErrorId 'AnErrorId' 
    } 
} 

If a function is written such that it writes a non-terminating error (using Write-Error), and generation of that error must be tested, the following pattern might be used:

function Invoke-Something { 
    [CmdletBinding()] 
    param ( )

  
    Write-Error 'Error' 
} 
Describe Invoke-Something { 
    It 'Throws a non-terminating error' { 
{ Invoke-Something -ErrorAction SilentlyContinue }| Should -Not -Throw 
{ Invoke-Something -ErrorAction Stop } | Should -Throw 
    }           
}