To get started with unit testing PowerShell code using the Pester-module, you need to be familiar with three keywords/commands:

• Describe: Defines a group of tests. All Pester test files needs at least one Describe-block.
• It: Defines an individual test. You can have multiple It-blocks inside a Descripe-block.
• Should: The verify/test command. It is used to define the result that should be considered a successful test.

Sample:

Import-Module Pester

#Sample function to run tests against    
function Add-Numbers{
    param($a, $b)
    return [int]$a + [int]$b
}

#Group of tests
Describe "Validate Add-Numbers" {

        #Individual test cases
        It "Should add 2 + 2 to equal 4" {
            Add-Numbers 2 2 | Should Be 4
        }

        It "Should handle strings" {
            Add-Numbers "2" "2" | Should Be 4
        }

        It "Should return an integer"{
            Add-Numbers 2.3 2 | Should BeOfType Int32
        }

}

Output:

Describing Validate Add-Numbers
 [+] Should add 2 + 2 to equal 4 33ms
 [+] Should handle strings 19ms
 [+] Should return an integer 23ms