Every tracking tool, be it Google Analytics, etracker, Piwik Pro always show at least one metric in their report displays. Thus, a report always consists of at least one dimension and also at least one metric.
Metrics are also called measurement values and show, how often a certain dimension occurred. Thus, metrics are always either percentage values or absolute numbers.
Within a tracking tool, metrics are usually always next to the dimension.
In web analytics, you need metrics to analyse how often dimensions occur. Examples of metrics and the appropriate dimensions are: