Where-Object

Filter input from the pipeline, control which objects will be passed along the command pipeline.
The '?' symbol and Where are both aliases for Where-Object.

Syntax
      Where-Object [-filterScript] {scriptblock}
             [-inputObject psobject] [CommonParameters]

Key
   -FilterScript scriptblock
       An expression that resolves to a Boolean (TRUE/FALSE) value.
       This will determine which input objects will be passed  along the command pipeline.

   -inputObject psobject
       The objects to be filtered. Typically objects are passed through the pipeline.

       When the -InputObject parameter is used to submit a collection of items,
       Where-Object receives one object that represents the collection.
       Because one collection object cannot be filtered, Where-Object returns the entire collection unchanged.

       To convert multiple items, pipe them to Where-Object.

       This parameter is an implementation detail: its purpose is to enable input via the pipeline, and its
       direct use with arrays (collections) does not (yet) provide any useful functionality.

Aliases: The '?' symbol and 'Where' are both aliases for Where-Object.
In some cases the .Where method may be faster.

Where-object determines which objects to pass along the pipeline by evaluating a script block that may include a reference to an object being filtered. If the result of the evaluation is True, the object being processed is passed along the pipeline; otherwise the object is discarded.

The scriptblock expression can use any of the PowerShell comparison operators as long as the result is a boolean.
Also: -not logical not (with ! as an alias) and -xor (Exclusive OR)

Comparison statement

A comparison statement, is a simplified syntax that can be used to filter the pipeline, neither the {brackets} or the pipeline placeholder $_ are required. Available in PowerShell 3.0 and greater.

Syntax
      command | Where test1 [conjunction test2] 

Key
   conjunction  Any of the following: (logical boolean operators) 
          -and, -or (Logical or), -bor (Bitwise or), -band (Bitwise and), -xor 

   Test1   An expression that resolves to a Boolean (TRUE/FALSE) value.

   Test2   An expression that resolves to a Boolean (TRUE/FALSE) value.
   comparison operators

As above, the expression can use any PowerShell comparison operators as long as the result is a boolean.
Also: -not logical Not (with ! as an alias) and -xor (Exclusive OR)

This only works with a single object property, and at most a single comparison.
Anything more complex still needs to be enclosed in a script block.

For example - to list files on drive f: greater than 1000000 bytes:
PS C:\> Get-ChildItem f:\ | Where-Object Length -gt 1000000

Which is equivalent to this scriptblock:
PS C:\> Get-ChildItem f:\ | Where-Object {$_.Length -gt 1000000}

You can also read properties in a smilar way:
PS C:\> (Get-ChildItem f:\).Length

You can’t use this simplified syntax to set properties.

To filter items that have a property set (not empty) just include the property name:

PS C:\> Get-ADuser -Filter * | Where-Object DisplayName

Examples

Get a list of files but exclude folders:

PS C:\> Get-ChildItem 'C:\Apps\' -Recurse | Where-Object {-not $_.PsIsContainer}

you could also do this with:

PS C:\> Get-ChildItem 'C:\Apps\' -file -Recurse

Get a list of all services that are currently stopped:

PS C:\> Get-Service | Where-Object {$_.Status -eq 'Stopped'}

Return the lines from file1.txt which do not also exist in file2.txt:

PS C:\> Get-Content file1.txt | where { (Get-Content file2.txt) -notcontains $_ }

List the processes with a working set greater than 25000K. (bytes = Kilobytes*1024):

PS C:\> Get-Process | Where-Object {$_.workingset -gt 25000*1024}

Get the processes with a ProcessName property that begins with the letter p. The -match operator enables you to use regular expressions:

PS C:\> Get-Process | Where-Object { $_.ProcessName -match '^p.*' }

“The enemy of art is the absence of limitations” ~ Orson Welles

Related PowerShell Cmdlets

PowerShell Syntax - Regular Expressions
ForEach-Object - Loop for each object in the pipeline.
Get-ChildItem - Get child items (contents of a folder or registry key).
Group-Object - Group the objects that contain the same value for a common property.
Select-Object - Select objects based on parameters set in the Cmdlet command string.
Sort-Object - Sort the input objects by property value.
Where (method) - Filter input from a collection.