Dimensions are the kinds of items you will be requesting values for from your Google Analytic’s data. For example: “Visitor Type” is a dimension with values of “New Visitor” or “Returning Visitor”.
Metrics, on the other hand, are the values that usually populate your report. So you might have a “Visitor Type” report, it shows the new and returning visitors, but it also gives you metrics: how many how many visits, % of new visits, Number of New visits, Bounce rate, Avg. Visit Duration, and so forth.