The difference between Dimensions and metrics is perhaps the first thing one needs to understand before diving deep into the web analytics reports.
Dimensions are ideally attributes associated with the visitor to your website. So this could be the city, the form type filled in, page names, referrers (Search engines, Direct, Social, External websites etc.)
Metrics are quantitative values associated with these dimensions. Hence, visits (sessions), page views, events, bounce rate etc. are metrics.
So, for e.g.: The dimension “City” may include New York as a row item. The associated visits (sessions) is the metric attributed to New York City.
To put it simply, dimensions shows up in the rows of Google Analytics or any web analytics reporting platform like Adobe Analytics. The numbers that are associated with these rows are metrics. Dimensions can be further broken down by associated dimensions to get a break up of the metrics attributed to each associated Dimension. So for E.g.: Visits to a specific Country can be further broken down by Cities to get a report on visits recorded from each city. Similarly a custom dimension can also be broken down by another to be correlated with the relevant metric.
However, one needs to be mindful of the fact that every metric cannot be associated with every dimension. Relevance is the key.