Generally speaking, API objects represent database tables that contain your organization's information. For example, the central object in the Salesforce data model represents accounts—companies and organizations involved with your business, such as customers, partners, and competitors. The term "record" describes a particular occurrence of an object (such as a specific account like "IBM" or "United Airlines" that is represented by an Account object). A record is analogous to a row in a database table.
Objects already created for you by Salesforce are called standard objects. Objects you create in your org are called custom objects. For a list of standard objects, see the Standard Objects.
While this document describes all of the objects available in the API, your applications work with only the objects that you are authorized to access. Programmatic access to objects is determined by the objects that are defined in your enterprise WSDL file, your organization configuration, your security access (which is configured by your organization’s system administrator in your personal profile), and your data sharing model. For more information, see Factors that Affect Data Access.
Most of the objects accessible through the API are read-write objects. However, there are a few objects that are read-only. This fact is noted in the description for the object.