A single campaign's report shows opens, clicks, bounces, unsubscribes, and (for connected e-commerce stores) revenue attributed to that specific send, broken down to the individual contact level so you can see exactly who clicked what. Mailchimp also tracks a per-contact "member rating," a rough engagement score based on open and click history over time.
The main reporting dashboard aggregates performance across campaigns and over time, useful for spotting a trend (declining opens, a particular subject-line style that performs better) without digging into each campaign's report individually.
Connecting Google Analytics lets Mailchimp automatically tag outbound links with UTM parameters, so clicks from a campaign show up correctly in your site analytics as their own traffic source, rather than blending into generic referral or direct traffic.