Monitoring your site is important to ensure that you notice developing problems before they emerge and can take pro-active action to minimise any problems which might affect your position in search engines or experience for your users.
There are lots of metrics that you could monitor - thousands in fact - but most people are too busy to dig into every nook and cranny and find out every piece of data.
For this reason, there are several ways to combine the data you need to monitor into one central location. Google Analytics has some great resources available in the form of custom segments and dashboards, which can be shared individually or as a package.
Deciding what to monitor really depends on your business/organisation and what is important to them. Ecommerce sites will have a very different interest to blogging and news sites, for example.
In this series of articles we'll consider some general metrics to monitor, along with some sector-specific metrics. These articles assume that you have Google Analytics running on your website, and you have associated a Google Webmaster Tools account with your Analytics account.