Combine an array into a string with semicolon separator 
$array = 'The','quick','brown','fox','jumps','over','the','lazy','dog.'
$array -join(';')
Details
Uses a join to combine the strings

Example
PS C:\> $array = 'The','quick','brown','fox','jumps','over','the','lazy','dog.'
>> $array -join(';')

The;quick;brown;fox;jumps;over;the;lazy;dog.
  |  |  
Combine an array into a string with space separator 
$array = 'The','quick','brown','fox','jumps','over','the','lazy','dog.'
$array -join(' ')
Details
Uses a join to combine the strings

Example
PS C:\> $array = 'The','quick','brown','fox','jumps','over','the','lazy','dog.'
>> $array -join(' ')

The quick brown fox jumps over the lazy dog.
  |  |  
Combine multiple strings 
$string1 = "The quick brown fox "
$string2 ="jumps over the lazy dog"
$string1 + $string2
Details
This should only be used for a fixed number of strings.
For combining a lot of strings or an unknown number, refer to the StringBuilder

Example
PS C:\> $string1 = "The quick brown fox "
>> $string2 ="jumps over the lazy dog"
>> $string1 + $string2

The quick brown fox jumps over the lazy dog
  
Composite Format String 
$string = 'The quick brown {0} jumps over the lazy {1}.'
$string -f 'fox','dog'
Details
Allows for the easy replacement of certain characters. Great when dealing with multiple languages.

Example
PS C:\> $string = 'The quick brown {0} jumps over the lazy {1}.'
>> $string -f 'fox','dog'

The quick brown fox jumps over the lazy dog.
  
Use StringBuilder to combine multiple strings 
$stringBuilder = New-Object System.Text.StringBuilder
for ($i = 0; $i -lt 10; $i++){
    $stringBuilder.Append("Line $i`r`n") | Out-Null
}
$stringBuilder.ToString()
Details
This performs much better the stanard array builder of +=

Example
PS C:\> $stringBuilder = New-Object System.Text.StringBuilder
>> for ($i = 0; $i -lt 10; $i++){
>>     $stringBuilder.Append("Line $i`r`n") | Out-Null
>> }
>> $stringBuilder.ToString()

Line 0
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9