A custom field can be of 6 different types and here's how they look like.
A text field that allows entering any type of character.
A number field allows entering any number in the text field.
The single option field is a drop down list that allows selecting only one option.
The multiple option field is also a drop down list but allows selecting more than one option at a time.
Allows selecting a date from a calendar.
A text field that allows entering multiple lines of text.
A number field that formats into a currency.
Allows entering a hyperlink to a file or web page in the Internet.
Allows enter an address that auto completes automatically from a database of addresses.
Is a drop down list of the contacts in the system.