Your site Getting Blacklisted and is omitted from search engine results...This can be due to "Duplicate content", "Keyword Stuffing" etc..
If you're trying to avoid a Google penalty, read this is from Google's site guidelines: Avoid hidden text or hidden links. Don't employ cloaking or sneaky redirects. Don't send automated queries to Google. Don't load pages with irrelevant words. Don't create multiple pages, subdomains, or domains with substantially duplicate content. Avoid "doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content.
Penalty usually refer to the punishment given by the search engines and it can be due to several reasons, like violating rules and regulations or doing black hat SEO etc. When your site got penalty its keywords SERP may goes down and your site traffic may be decreased rapidly etc etc.
Penalty is a way of punishing websites when they go against search engine rules. A website can be penalized for many reasons such as duplicate contents, keyword stuffing, content farming and cloaking. It can be any search engine not just Google.
Penalty means you are being punished. So if your site caught by doing unethical activities the your site might be penalized. It includes: - Keyword stuffing - Duplicate content - Mirror website - Link farm - Hidden affiliate links - Cloaking and doorway pages; etc