ESPAsyncTCP
Async TCP Library for ESP8266 Arduino
For ESP32 look HERE
This is a fully asynchronous TCP library, aimed at enabling trouble-free, multi-connection network environment for Espressif's ESP8266 MCUs.
This library is the base for ESPAsyncWebServer
AsyncClient and AsyncServer
The base classes on which everything else is built. They expose all possible scenarios, but are really raw and require more skills to use.
AsyncPrinter
This class can be used to send data like any other Print interface (Serial for example).
The object then can be used outside of the Async callbacks (the loop) and receive asynchronously data using onData. The object can be checked if the underlying AsyncClientis connected, or hook to the onDisconnect callback.
AsyncTCPbuffer
This class is really similar to the AsyncPrinter, but it differs in the fact that it can buffer some of the incoming data.
SyncClient
It is exactly what it sounds like. This is a standard, blocking TCP Client, similar to the one included in ESP8266WiFi