Dataverse it is a service for storing data in the could. It is the back-end to Model-Driven Apps and by that also to some of the Dynamics apps e.g. Dynamics Sales and Dynamics Customer Service (former CRM-parts in Microsoft Dynamics CRM). Dataverse is more than just storage in the cloud. It also comes with concepts like security (configure security models for your apps), business logic and data integration.
You can get an overview of the tables under Data, but you should make it a habit not to create new tables under Data, but rather work within Solutions. It is not recommended to work within the Default Solution.
Microsoft Dataverse originates from Microsoft Dynamics CRM. As Microsoft's CRM/xRM platform evolved into Power Platform concepts like editing existing data models, creating new data models and managing security for your application was transformed into the Common Data Service (CDS). One explanation for the origin of CDS is that Microsoft saw similarities in their different products, AX (Finance & Operations) had something called Accounts, Dynamics CRM/365 CE had Accounts etc. and in order to make it easier to talk about and implement integration between different products the CS was introduced. It came with a pre-defined set of entities in what is called Common Data Model (CDM).
In November 2020 Common Data Service (CDS) was rebranded into Dataverse.