Every report is based on a combination of 'Dimensions' and 'Metrics'. Each serve a specific function:
- Dimensions: describe data -- think of them as qualitative. They are usually green when selecting them when setting up your reports.
- Metrics: measure data -- think of them as quantitative. They are usually blue when selecting them when setting up your reports.
Each are usually used together to create your report. "The values of dimensions and metrics and the relationships between those values is what creates meaning in your data." It would be unhelpful to look at a dimension like 'Pages' without at least one metric like 'Views' to give it context.
More information can be found on Google's Dimensions and Metrics explanation page.