Flink offers different levels of abstraction for developing streaming/batch applications.
The lowest level abstraction simply offers stateful streaming. It is embedded into the DataStream API via the Process Function. It allows users freely process events from one or more streams, and use consistent fault tolerant state. In addition, users can register event time and processing time callbacks, allowing programs to realize sophisticated computations.
In practice, most applications would not need the above described low level abstraction, but would instead program against the Core APIs like the DataStream API (bounded/unbounded streams) and the DataSet API (bounded data sets). These fluent APIs offer the common building blocks for data processing, like various forms of user-specified transformations, joins, aggregations, windows, state, etc. Data types processed in these APIs are represented as classes in the respective programming languages.
The low level Process Function integrates with the DataStream API, making it possible to go the lower level abstraction for certain operations only. The DataSet API offers additional primitives on bounded data sets, like loops/iterations.
The Table API is a declarative DSL centered around tables, which may be dynamically changing tables (when representing streams). The Table API follows the (extended) relational model: Tables have a schema attached (similar to tables in relational databases) and the API offers comparable operations, such as select, project, join, group-by, aggregate, etc. Table API programs declaratively define what logical operation should be done rather than specifying exactly how the code for the operation looks. Though the Table API is extensible by various types of user-defined functions, it is less expressive than the Core APIs, but more concise to use (less code to write). In addition, Table API programs also go through an optimizer that applies optimization rules before execution.
One can seamlessly convert between tables and DataStream/DataSet, allowing programs to mix Table API and with the DataStream and DataSet APIs.
The highest level abstraction offered by Flink is SQL. This abstraction is similar to the Table API both in semantics and expressiveness, but represents programs as SQL query expressions. The SQL abstraction closely interacts with the Table API, and SQL queries can be executed over tables defined in the Table API.
This concepts section explains the basic concepts behind the different APIs, that is the concepts behind Flink as a stateful and timely stream processing system.