A microservice is for all intents and purposes a small restful web service :)
It is an approach to developing a single application as a suite of small services, each running in its own process and communicating with lightweight mechanisms, often an REST API - martinfowler.com 
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So-so panel on microservices. See microservices.io
A microservices-based architecture:
- Naturally enforces a Modular programming
- Lends itself to a continuous delivery
- Adheres to Service granularity principle
- Domain-driven design
- IDEAL architectures
- Polyglot (computing) programming
- Container Deployment
- Decentralized continuous delivery
- DevOps with holistic service monitoring - hsr.ch
- Beneficial to scalability -Microservices: pdf
These services are built around business capabilities and independently deployable by fully automated deployment machinery. There is a bare minimum of centralized management of these services, which may be written in different programming languages and use different data storage technologies.
# See also
- Microservice post on martinfowler.com
- Fallacies of Distributed Computing
- Microservices not a free lunch - highscalability.com
- Serverless Microservices
- Microservices Visualized
- Ontology v. Stigmergy
- Constructing Explanations