Martin Fowler explains:
Let's get some background on the way it has evolved over the years. Enterprise architecture evolved more from historic mainframe computing, through client-server architecture (two-tier to n-tier) to Service-Oriented Architecture (SOA).
The transformation from SOA to microservices is not a standard defined by an industry organization, but a practical approach practiced by many organizations. SOA eventually evolved to become microservices.
Adrian Cockcroft, a former Netflix Architect, describes it as:
Similarly, the following quote from Mike Gancarz, a member that designed the X Windows system, which defines one of the paramount precepts of Unix philosophy, suits the microservice paradigm as well:
Microservices shares many common characteristics with SOA, such as the focus on services and how one service decouples from another. SOA evolved around monolithic application integration by exposing API that was mostly Simple Object Access Protocol (SOAP) based. Therefore, middleware such as Enterprise Service Bus (ESB) is very important for SOA. Microservices are less complex, and even though they may use the message bus it is only used for message transport and it does not contain any logic. It is simply based on smart endpoints.
Tony Pujals defined microservices beautifully:
Though Tony only talks about the HTTP, event-driven microservices may use the different protocol for communication. You can make use of Kafka for implementing the event-driven microservices. Kafka uses the wire protocol, a binary protocol over TCP.