A custom object is created by an administrator to extend you organization data based on business requirements.
It has the same functionality as a standard object such as creating custom fields, page layouts, record types and validation rules, etc.
Attention: When a custom object is created, by default only the standard system administrator has CRED permissions, and the org-wide default values are Public Read/Write. It is therefore important to set the security.
Limited to 200 custom objects in the Enterprise Edition.