Final answer:
The command 'docker-compose build' is used for building or rebuilding services based on a 'docker-compose.yml' file, while 'docker-compose up' is used to start and run the entire application. If the images do not exist or are outdated, 'docker-compose up' will build them before starting the containers.
Step-by-step explanation:
The difference between docker-compose up and docker-compose build lies in their purpose and function within the Docker ecosystem. docker-compose build is a command that specifically builds or rebuilds services in a docker-compose.yml file. This step is necessary when you want to create or update the images that your containers will be based on. On the other hand, docker-compose up is a command used to start and run your entire multi-container application as defined in the docker-compose.yml file. When you run docker-compose up, Docker will check whether the images for all services exist and are up-to-date. If they are not, Docker will automatically execute a build process before starting the containers. Therefore, docker-compose up can include the build process implicitly if required, but it is primarily meant to launch the application.