Website monitoring is the process of testing or tracking (monitoring) how end-users interact with a website or web application. Website monitoring is often used by businesses to ensure that their customers are able to access their online applications and perform actions such as searching, online shopping, checking an account balance, or simply researching.
Monitoring is essential to insure 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 an application needs to function like network connectivity, Domain Name System records, database connectivity, bandwidth, computer resources including free RAM, CPU load, disk space, events, etc. Commonly measured metrics are response time and availability (or uptime), although both consistency and reliability metrics are starting to gain popularity.
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. External performance monitoring is also known as end-user monitoring or end-to-end performance monitoring.
Real user monitoring measures the performance and availability experienced by actual users, diagnoses individual incidents, and tracks the impact of a change.
Source: Wikipedia