When standard fields are not enough to cover all your needs, you can create new fields called custom fields. The number of custom fields allowed per object varies according to your Salesforce Edition (Ex: on the Enterprise Edition, it is possible to create up to 500 fields per object).
You can create fields using the Custom Field Creation Wizard by choosing the type of field you want to create, setting the field attributes (label, picklist values, description and help text), then the field-level security and finally choosing the page layouts that will display the field.
The field types define the technical characteristics of the field (checkbox, picklist, number, formula, etc.).
Each field is associated with an API name that will be recognizable thanks to its extension « __c » (c = custom). Attention when creating, API names do not support special characters (é, è, à, ‘, …), they are replaced by « _ »