CI refers to Continuous Integration, the practice of everyone pulling and pushing in the same branch in order to ensure that all the code is always integrated into the latest state as explained in these links:
An automated build is when you use a task runner like Jenkins, Travis or CircleCI to automatically execute all the steps that are needed for your build instead of someone executing those by hand. A good example is ensuring the tests pass and then generating a Docker image and pushing it into the register.
And in summary...
Most people doing CI use builds, but not all people using builds are doing CI.
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