Strings can be entered with " as delimiters:
"Hello world!"
As you can see, quotation marks are not printed in the output by default. This can be changed by using InputForm:
InputForm["Hello world!"]
Strings can be joined using <>:
"Hello" <> " " <> "world!"
Numbers cannot be joined to strings:
"Debian" <> 6
They have to be converted to strings using ToString first:
"Debian" <> ToString[6]