Selector Widgets are a group of widgets that let you create report-level settings. They let you select the account/campaign/label which you would like to report on for your choice of the date range. This segment also offers to add custom metrics using Calculated Metrics.
You can read the details of this widget in this help article.
The Layout widgets include title, subtitle, text box, and image box, which you can include in your report to customize.
Use Chart Widgets to report on the performance of metrics for different segments and/or over a period of time:
Monitor contribution of impressions, clicks, conversions, etc received on different segments like campaigns, ad group, network, etc.
Report on absolute performance numbers of accounts by campaigns, ad groups, network, and devices.
The Table widgets contain different types of widgets that let you report on key performance metrics in an easy-to-read tabular format. You can report on the performance of campaigns, ad groups, keywords, ads, and more.
Use the Top Elements widgets to report on top items, such as keywords, text ads, image ads, campaigns, ad groups, and search terms.