You can add these items as Custom Fields to your Events and Contact Details.
Each one works a little differently:
Checkbox
The user can select or not select the box. It is an optional selection.
Group of Checkboxes
The user can select none or all or some of the options.
These would be used for something that is optional but that also allows for multiple choices.
Radio Buttons
The user must select one of the options.
Dropdowns
Similar to the Radio Buttons, but in the case of Dropdowns, the first item, the "default" value, is blank.
Note: Each of the options Checkbox, Group of Checkboxes, Radio Buttons and Dropdown box fields also allows you to create a Smart Group based on selection.