Types of error

Briefly on HTTP Status codes

Every time you visit a website your browser receives a message from the server that hosts the website. This message is called a HTTP Status code. As a SEO it is necessary to understand what the most important codes mean.
200 OK - The request has succeeded. This is the standard response for successful HTTP requests.
301 Moved Permanently - The requested resource has been assigned a new permanent location. This and all future requests should be directed to the given location. This status code is used for 301 redirects. In most instances, the 301 redirect is the best method for implementing redirects on a website. A 301 redirect will pass most, if not all the linkjuice from the original location.
302 Found - The requested resource resides temporarily at a different location. By using a 302 redirect instead of a 301, search engines will know that this is only a temporary state. No appreciable amount of linkjuice will be passed.
404 Not Found - The server has not found anything matching the requested location. No indication is given of whether the condition is temporary or permanent. In time, the page will be removed from the search engine's index.
503 Service Unavailable - The server is currently unavailable (this could be due to overload or maintenance). Search engines will know that this is a temporary state. This status code should be used when taking down a site for maintenance.
You can read more about HTTP status codes here.