Difference between the API interfaces in Api and Api/data folders?

Question

API interfaces are available in Api and Api/data folders,

what is the basic difference between these two api types?

Answer 1

Data interfaces

Data interfaces are used to maintain the integrity of the data, data interfaces defines all the setters and getters for the related entity. So even if there is any changes in the model or business logic you will always get consistent data. Data interfaces resides in VendorName\ModuleName\Api\Data, so in all the modules you can found data interfaces in Api/Data folders, related model need to implement these interfaces and provide the setters, getters deffinition.

Interfaces

Repository interfaces give access to persistent data entities. interfaces have the following methods:

save(data entity interface): Creates a new record if no id present, otherwise updates an existing record with the specified id.

get(id): Performs a database lookup by id and returns a data entity interface (such as CustomerInterface or AddressInterface).

getList(search criteria): Performs a search for all data entities matching the search criteria and returns a search results interface to give access to the set of matches.

delete(data entity interface): Deletes the specified entity (the key is in the entity).

deleteById(id): Deletes the specified entity when you only have the key for the entity.

You can check one Example from here : https://webkul.com/blog/magento2-service-contract/

Answer 2

They are all part of a new concept in Magento 2 called Service Contracts.

Data models are intended to map entities, e.g. A customer, product, category. They should only contain properties used to describe those entities and methods used to set/get that data (mutator/accessor methods). They should not have any logic which define how the Data Model is saved/loaded however (that responsibility is handed to Repository Models) or any behavioural logic, e.g. Password reset methods (that is handled in the Management Interfaces).

A Repository Model defines common CRUD operations (Create, Replace, Update, Delete) for models. Though Repository interfaces do not extend from a common interface, they do share some common methods, e.g. save, getById, getList and delete, which are self-explanatory.

API models are better described as 'Management' Models. Logic which is relevant to an entity but which is not related to setting/getting data is encapsulated here. For example, for a customer entity, you would want to add some logic which handles passwords (e.g. Password validation, resetting passwords, etc). This logic goes in the Management model.

Differences between Magento 1 and Magento 2

To save an entity, the data model is passed to the save method of the repository. This is different to Magento 1, where the Data Model had it's own load/save methods which were used to load and save the entity.

Magento 1 collections do not exist in the same fashion in Magento 2. Instead, calling the getList method of a repository will return an array of Data Models. This array can be filtered by passing a SearchCriteria object to the getList method, which is analogous to calling addFieldToFilter, AddAttributeToSelect, etc methods on a collection object in Magento 1.

Under the hood, if you dig deep enough, you'll actually find that a lot of these Magento 1 ORM concepts are still present in Magento 2, which means the Service Contract layer is more of a wrapper for the old Magento 1 logic. This is because Magento 2 is basically a rewrite of Magento 1. Using the interface-driven approach of defining classes means that a lot of the refactored Magento 1 code can be hidden behind the shiny new Service Contract interfaces, whilst the core team (or community) refactors the underlying Magento 1 code (It is Magento's stated intention to get rid of all the refactored Magento 1 code eventually).

Source: https://devdocs.magento.com/guides/v2.2/extension-dev-guide/service-contracts/service-contracts.html