Your place in the search results i.e. your search ranking reflects your online reputation. If your website shows up at the first page (or most preferably, is in the first 3 ranks), your website is considered by the search engine to be the most authoritative as well as most relevant site, out of all the millions of sites being indexed. Online reputation is very hard to build up, but actually easy to destroy. Here are a few things to be on guard for.

Spam comments

Many of the major search engines will give penalties if your website contains spam keywords. While you may not be using them when you're creating the content of your website, blogs and forums frequently have a comment facility that permits other people (spammers included) to add content.

Spam comments must be avoided not just because of search engine penalties, but because it portrays your website as unprofessional. It also detracts from the reader experience (no one likes to read spam something spammers actually need to learn). Add a captcha or moderate your comments to prevent this.

Doorway pages

Doorway pages are pages whose only role is to redirect a reader to another page (generally not at all connected to the content of the doorway page). For instance, a doorway page may contain lots of words for gardening that a searcher may put as keywords; but, the page itself doesn't contain any pertinent content on gardening and will merely link to some insurance website.

Doorway pages may have been very effective (though still not very useful) back in the early hours of search engines, as their algorithms were simplistic and simply used page contents as the only influencer of page rank. Search engines have evolved in the intervening time, and these types of pages are frowned upon.

Hidden text and links another holdover from the premature days of search engines, hidden text and links are those that have a foreground color that is very similar as the background color. The idea (albeit misguided) here is that search engines will just see the text from a web page, and pays no attention to color. So, only search engines will see the hidden text and links, and people with browsers will not see them (and will probably think the website is not spamming). For sure, search engines have evolved since then, and will give penalties to a website that uses such techniques.

Malware on your website

Having malware on your website will frequently send a red flag to search engines and will neither give a penalty to your site's ranking nor will cause a warning page to pop up before the searcher is directed to your website. Even so, having malware on your website detracts from the user experience - you may lead the user the first time, but he is very much improbable to return.

You may not be (hopefully) installing malware on your website intentionally, but there are still ways to have it on your website inadvertently. If you're running a blog or a forum, you have to update your software often - at times this software will contain vulnerabilities that allow people with questionable intent to take control of your website and attach malware to your WebPages.