Exploring standard, custom, external, and big objects
Salesforce objects can broadly fit into one of four types. While you will no doubt have worked with the standard and custom object functionality available on the platform, it is necessary to understand how they work at a lower level to truly understand what it means to issue SOQL queries against those object types and how it affects performance. Secondly, you may not have worked with external objects and big objects before. These are different when it comes to data storage and access (external object data isn't stored on the platform and is accessed on-demand through an external data source. Big object data ensures consistent performance but data lives in a separate location to standard/custom objects within the Salesforce platform).
sObjects
An sObject is the generic form of any Salesforce Object. The standard Salesforce objects and the custom objects you create are concrete types of sObjects (they all inherit common...