Information about Non Routable networks

Hello Everyone,
I have a simple question for all of you. Please anyone tell me what are the networks which are Non Routable with the help of examples. Whether there are any security issues involved in the Non Routable networks? Or if there is any risk of security in the Non Routable networks ; then how can we resolve these risks? Whether there are any tools we can use to avoid the risk?
Please give example with the answers.
Thanks
