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.

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 Length -gt 1000000

Which is equivalent to this scriptblock:
PS C:\> Get-ChildItem f:\ | where {$_.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 DisplayName

Examples

Get a list of files but exclude folders:

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

Get a list of all services that are currently stopped:

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

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

PS C:\> Get-process | ? {$_.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.