In your phone there's a built-in phonebook where you can store numbers for your friends and colleagues. Adding the information in Contacts makes it easily accessible, and you don't have to type in the digits when calling or sending messages.
In most phones you can also add other details, such as addresses and birthdays, or add a specific ringtone or picture for a specific contact. You can reach your stored contacts from the Contacts menu, or from standby by pressing and holding the first letter in the name of the contact.
To add a contact:

