The Operating System (OS) piece of software is the most important piece of software on any computer system. It is important as this is the main piece of software on your computer that recognises user keyboard and mouse input. The operating system is able to recognise what is being input and perform certain tasks based on the input.
Operating Systems are used to allow the user to navigate throughout their computers files using given inputs in a graphical way. A good example of a select few operating systems are: Windows, OSX Lion, Linux and many more such as Ubuntu perhaps.
Utility programs are pieces of software that you can run on your computer to improve the running of your personal system. There may be some bad/junk files on your computer that are slowing your system down, the easiest way to get rid of these types of files and prevent yourself from receiving these files in the future is using system utilisation software that cleans out your drives of any bad files.
Library programs are programs that are often called a library because they hold various different resources. There would be programs that require resources from a given library and would need to have access to these resources whenever required.
Translator programs are pieces of software that run on your computer whenever instructed to, the software will read what is displayed on screen (most likely a browser application reading a browsers text) and modify what HTML content is displayed on the screen based on a variety of a functions that are called dependent on what language the user wants the program to translate the text into.