What is the difference between Dimensions & Metrics in Google Analytics?

As a marketer you usually will get one or more metrics for one dimension. A dimension can be measured in several metrics, but not the other way around. That's why the metrics are the columns, the dimensions are the rows in every report table.

A metric is usually something you can count: 10 or 100 Visits, 1,000 or 10,000 Pageviews, 400 Total Events.

A dimension is what you are applying the metric to: the Page Title, the Page Path, the Event Label. 

So for example, the dimension 'Page Title' can be analyzed via the metrics 'Pageviews', 'Unique Pageviews', 'Time on Page', 'Exit Rate' and so on. So a metric is a way to look at a dimension.

Hope this i helpful.

To understand the specifics of Dimensions & Metrics please visit the Google developers reference guide:
https://developers.google.com/analytics/devguides/reporting/core/dimsmets

Cheers - SRK Iyer