Salesforce - Dashboard

Few key features of Dashboard

Reports provide all the data shown in a dashboard. Dashboards can show data from more than one report.
When refreshing a dashboard, all the data-providing reports must run. If the reports take a long time to run, then the dashboard does too.
Dashboards are shared via folders. Whoever has permission to the folder your dashboard is saved in also has access to your dashboard. Ensure that you save your dashboard in an appropriate folder.

Each dashboard can have up to 3 filters. Contact Salesforce to increase the filter options limit in Salesforce Classic. A maximum of 50 filter options is possible.
You can’t add filters to dashboards that contain Visualforce or s-control components.
It’s not possible to filter on bucket fields. However, it is possible to use a report filtered on a bucket field on the dashboard page.
Filters aren’t applied when you schedule or email a dashboard.
You can’t filter data on a joined report in dashboard view or add a filter to a dashboard that only has joined reports.
You can’t use custom summary formulas in dashboard filters.
In Lightning Experience, relative dates (like “last month”) don’t work in dashboard filters.
Filters on a dashboard allow you to choose different views of data. After you filter a dashboard, the filtered view is preserved so that the next time you see the dashboard, data is filtered in the same view. Without dashboard filters, you'd have to create multiple dashboards, each with its own set of filtered reports.

Your organization can have up to 5 dynamic dashboards for Enterprise Edition, 10 for Unlimited and Performance Edition, and 3 for Developer Edition.
ou can't save dynamic dashboards to personal folders.
You can't schedule refreshes for dynamic dashboards. They must be refreshed manually.
Dashboard components that use Visualforce ignore the running user; content displays only if the viewing user has access to the Visualforce page. Other components in the dashboard are not affected.

The data is as current as the date and time displayed after As of... at the top right corner of the dashboard.
When the dashboard has finished refreshing, Salesforce 1 tells you that the dashboard is ready through the right-hand notification panel. Since push notifications aren’t enabled for dashboard refreshes, you're notified only in the app and not in through email.
Additional refreshes submitted during a refresh and up to one minute after the completion of a refresh are ignored; users view the most current data.
When you change filters on a dashboard, the dashboard shows previously cached data, if it exists. If no data exists, the dashboard fetches the latest. Either way, the dashboard’s “Last Refreshed” date shows you the timestamp for the data you’re viewing. Click Refresh to get the most recent data.
If your dashboard data doesn't refresh after ten minutes, the refresh automatically stops. If necessary, click Refresh again.

References

Salesforce Technical Diary