function myFunction
{
    param(
        # If parameter 'a' is used, then 'c' is mandatory
        # If parameter 'b' is used, then 'c' is optional, but allowed
        # You can use parameter 'c' in combination with either 'a' or 'b'
        # 'a' and 'b' cannot be used together

        [parameter(ParameterSetName="AandC", mandatory=$true)]
        [switch]$a,
        [parameter(ParameterSetName="BandC", mandatory=$true)]
        [switch]$b,
        [parameter(ParameterSetName="AandC", mandatory=$true)]
        [parameter(ParameterSetName="BandC", mandatory=$false)]
        [switch]$c
    )
    # $PSCmdlet.ParameterSetName can be used to check which parameter set was used
    Write-Host $PSCmdlet.ParameterSetName
}

# Valid syntaxes
myFunction -a -c
# => "Parameter set : AandC"
myFunction -b -c
# => "Parameter set : BandC"
myFunction -b
# => "Parameter set : BandC"

# Invalid syntaxes
myFunction -a -b
# => "Parameter set cannot be resolved using the specified named parameters."
myFunction -a
# => "Supply values for the following parameters:
#    c:"

When you want for example enforce the use of the parameter Password if the parameter User is provided. (and vise versa)

Function Do-Something
{
    Param
    (
        [Parameter(Mandatory=$true)]
        [String]$SomeThingToDo,
        [Parameter(ParameterSetName="Credentials", mandatory=$false)]
        [String]$Computername = "LocalHost",
        [Parameter(ParameterSetName="Credentials", mandatory=$true)]
        [String]$User,
        [Parameter(ParameterSetName="Credentials", mandatory=$true)]
        [SecureString]$Password
    )

    #Do something
}

# This will not work he will ask for user and password
Do-Something -SomeThingToDo 'get-help about_Functions_Advanced' -ComputerName

# This will not work he will ask for password
Do-Something -SomeThingToDo 'get-help about_Functions_Advanced' -User

Function Do-Something
{
    Param
    (
        [Parameter(Mandatory=$true)]
        [String]$SomeThingToDo,
        [Parameter(ParameterSetName="Silently", mandatory=$false)]
        [Switch]$Silently,
        [Parameter(ParameterSetName="Loudly", mandatory=$false)]
        [Switch]$Loudly
    )

    #Do something
}

# This will not work because you can not use the combination Silently and Loudly
Do-Something -SomeThingToDo 'get-help about_Functions_Advanced' -Silently -Loudly