Yes, you can create customized fields for your Accounts, Contacts, and Opportunities.
- Go to "Settings" > "Customize fields".
- Select the entity you want to add custom fields to by clicking on either the "Account", "Contact", or "Opportunity" tabs in the top bar.
- Click on the large orange "+" button at the bottom right of the screen to create a custom field.
- Select the right field type and give it a descriptive name.
Learn more on what predefined custom fields are for.