A custom field can be added to standard objects that support custom fields and custom objects to extend the capabilities of Salesforce to suit the specific need

A custom field can be added to standard objects that support custom fields and custom objects to extend the capabilities of Salesforce to suit the specific needs of an organization. Custom fields can be accessed in code by referring to them by their API Name which is their specified Field Name value suffixed with an __c.
Attribution to: Peter Knolle
This content is remixed from stackoverflow or stackexchange. Please visit https://salesforce.stackexchange.com/questions/4975