if(condition){
   <statement block>
}
elseif(condition2)
{
   <statement block>
}
else
{
   <statement block>
}

If condition evaluates to $true, then PowerShell executes the statement block you provide. Then, it resumes execution at the end of the if / elseif / else statement list. PowerShell requires the enclosing braces around the statement block even if the statement block contains only one statement.

If condition evaluates to $false, then PowerShell evaluates any following (optional) elseif conditions until one matches. If one matches, PowerShell executes the statement block associated with that condition, then resumes execution at the end of the if / elseif / else statement list.

If none of the conditions evaluate to $true, then PowerShell executes the statement block associated with the (optional) else clause, then resumes execution at the end of the if / elseif / else statement list.

switch [options] (expression)
{
   <comparison value> { <statement block> }
   -or-
   { expression }     { <statement block> }

   (…)
   default            { <statement block> }
}

or:

switch [options] –file <filename>
{
   <comparison value> { <statement block> }
   -or-
   { expression }     { <statement block> }

   (…)
   default            { <statement block> }
}

When PowerShell evaluates a switch statement, it evaluates expression against the statements in the switch body. If expression is a list of values, PowerShell evaluates each item against the statements in the switch body. If you specify the –file option, PowerShell treats the lines in the file as though they were a list of items in expression.

The <comparison value> statements allow you to match the current input item against the pattern specified by <comparison value>. By default, PowerShell treats this as a case-insensitive exact match, but the options you provide to the switch statement can change this.

?        Any single unspecified character
*        Zero or more unspecified characters
[a-b]    Any character in the range of a-b
[ab]     The specified characters a or b

The { expression } statements allow you to process the current input item (stored in the $_ variable) in an arbitrary script block. When it processes an { expression } statement, PowerShell executes the associated statement block only if { expression } evaluates to $true.

PowerShell executes the statement block associated with the (optional) default statement if no other statements in the switch body match.

When processing a switch statement, PowerShell tries to match the current input object against each statement in the switch body, falling through to the next statement even after one or more have already matched. To have PowerShell exit a switch statement after it processes a match, include a break statement as the last statement in the statement block.

For example:

$myPhones = "(555) 555-1212","555-1234"

switch -regex ($myPhones)
{
  { $_.Length -le 8 } { "Area code was not specified"; break }
  { $_.Length -gt 8 } { "Area code was specified" }
  "((555)).*"       { "In the $($matches[1]) area code" }
}

Produces the output:

Area code was specified
In the 555 area code
Area code was not specified

By default, PowerShell treats this as a case-insensitive exact match, but the options you provide to the switch statement can change this.