The member of sqaforums are a helpful bunch but can become less helpful if it appears that the question poser has not done much research. Google returned 30 million hits on your question "what is structured systems testing?"
But let us break down your question a little. First I would suggest looking up "system testing" on say wikipedia. That will give you a start. Then think about how the word "structured" could be applied to that concept.
If this is just an interview question then I'd recommend not going to that interview. If you can't find out a definition for something like that then I think that perhaps testing is not the right profession for you. IMO testing requires the kind of mind that likes asking difficult questions and figuring out the answer by yourself. But then education systems never help that much when all they expect you to do is regurgitate an answer they told you earlier. But that is a rant for another place and time.
Well I tried to teach a person to fish today.
Sometimes there's a buggy. How many drivers does a buggy have? So let's just say that I'm driving this buggy, and if you fix your attitude, you can ride along with me. -The Cowboy, Mulholland Drive