Websites which have nothing informative or just full of ads are considered as spamming websites. These websites are usually optimized in a smart way (by employing grey hat and black hat techniques) so that they appear high in search results despite of having nothing on the website.
Anything against Google's guidelines is spam. Hence the spamy site is such which violates Google's guidelines.
If you are not going to follow the general guidelines provided by search engines, they will automatically take your website for spamming. Things like using hidden text, useless or copied content, cloaking etc are the activities that are not allowed by search engines.