They are all closely related. I look at microservices as an evolution (or even category) of service oriented architecture. SOA and microservices are both looking to deliver applications in a component-based way. Historically, SOA architecture would rely on an Enterprise Service Buses (ESB) to pass messages between the services. With modern microservices, they are often standalone REST APIs. For example, an authentication microservice could be a REST API that takes an HTTPS request with some parameters, validates credentials, and returns some token for authentication.