Introducing custom fields
Let's say you want to store a person's birthday.
Enter "Birthday" in the title field and select "Date" as the field type.
Click the "For People" checkbox and then click the "Add Custom Field" button.
Now, head over to a person and you will see a new button titled "+ Custom Field". Click on it and select "Birthday" and enter the date in the field then click on the save button.
Every time you view this person you will see their birthday displayed.
To view all the people who you have added a birthday for, go back to "Setup > Custom Fields" and click on the birthday one. Then click on the "People" tab.