From Seo Wiki - Search Engine Optimization and Programming Languages
Website monitoring is the process of testing and verifying that end-users can interact with a website or web application. Website monitoring is often used by businesses to ensure that their sites are live and responding.
Website Security monitoring is also used to verify that a domain (and web site) is not only responding properly, but has not been hacked, blacklisted or hijacked.
Multiple tools are available to automate site availability checks and security checks.
Monitoring is essential to ensure that a website is available to users and downtime is minimized. Users that rely on a website or an application for work or pleasure will get frustrated or even stop using the application if it is not reliably available. Monitoring can cover many things that an application needs to function, like network connectivity, Domain Name System records, database connectivity, bandwidth, and computer resources like free RAM, CPU load, disk space, events, etc. Commonly measured metrics are response time and availability (or uptime), but consistency and reliability metrics are gaining popularity.
Inside or outside monitoring
Website monitoring can be done from both inside and outside of a corporate firewall. Traditional Network Management solutions focus on inside the firewall monitoring, whereas external performance monitoring will test and monitor performance issues across the Internet backbone and in some cases all the way to the end-user.
Inside firewall monitoring is done by special hardware appliances which can help you determine if your internal applications’ sluggish performance is caused by: design of applications, internal infrastructure, internal applications or connections to any public internet. 
External performance monitoring is also known as end-user monitoring or end-to-end performance monitoring.citation needed
Real user monitoring measures the performance and availability experienced by actual users, diagnoses individual incidents, and tracks the impact of a change.
|If it's up...||A.K.A...||It's down... per year|
|99%||two 9's||87 hours, 36 minutes|
|99.9%||three 9's||8 hours, 45 minutes, 36 seconds|
|99.99%||four 9's||52 minutes, 33.6 seconds|
|99.999%||five 9's||5 minutes, 15.36 seconds|
|99.9999%||six 9's||31.68 seconds|
Various types of protocol
Website monitoring service can check HTTP pages, HTTPS, FTP, SMTP, POP3, IMAP, DNS, SSH, Telnet, SSL, TCP, PING, Domain Name Expiry, SSL Certificate Expiry and a range of other ports with great variety of check intervals from every 4 hours to every one minute. Typically, most website monitoring services test your server anywhere between once-per hour to once-per-minute.
Time performances: for example an HTTP page should answer in less than 1 second (for a download of 16Ko) to be considered as "good".citation needed
Various type of monitoring
You may monitor a single page of your website, but you can also monitor a complete business process (often referred to as multi-step transactions).
Servers around the globe
Website monitoring services usually have a number of servers around the globe - USA, Europe, Asia, Australia and other locations. By having multiple servers in different geographic locations, monitoring service can determine if a Web server is available across different Networks worldwide. Some vendors claim that the more locations the better picture on your website availability while others say that three globally distributed stations are sufficient and more stations do not give more information.
Types of website monitoring
There are two main types of website monitoring
- Synthetic monitoring also known as active monitoring, and
- Passive monitoring also known as real monitoring.
Notification options - alerts
As the information brought by website monitoring services is in most cases urgent and may be of crucial importance, various notification methods, often known as "alerts" are used: e-mail, IM, regular and cell phones, SMS, fax, pagers, Skype, etc.
- Synthetic monitoring
- Real user monitoring
- Internet Server Monitoring
- Web analytics
- Website tracking
- Application Performance Management
- Network Management
- Network monitoring