The property inspector
The Property Inspector allows you to set some common properties for controls, such as alignment, position and size as well as specific settings for certain controls (selection state, text properties, color, etc.).
You can toggle the Property Inspector with the keyboard shortcut CTRL/⌘+; or ALT/OPTION+ENTER. When hidden you can also toggle it by right-clicking a control and selecting "Properties...".
If you have multiple controls selected it will display properties that are common to all the selected controls, as well as options for aligning and distributing the selection.
When no controls are selected, the Property Inspector shows notes for the selected wireframe.
Note: You can format notes text using the syntax described here to make text bold, underlined, etc.
For Symbols, it also shows the count and name of wireframes where the selected object is used.