What is CI?
CI stands for Continuous Integration. It is a phase of the software development cycle on which the code written by all the team members is merged or integrated in to one shared branch. The CI workflow is a script with a series of commands that will be executed one after another. This script contains steps that the team decided will be used to check the quality of the code being produced. It may include things like verifying for possible bugs, verifying if the code formatting and styling follows what the team has decided will be used, or running automated tests like unit or integration tests.
What is CD?
CD may stand for either Continuous Delivery or Continuous Deployment.
Continuous delivery automates the release of the code validated during the CI phase. The main objective of the continuous delivery phase is to always have code ready for deployment in production. During this step the code goes through the build, packaging, and other operations to produce the deployable release.
This step triggers the next which is the continuous deployment.
Continuous deployment is the part that releases the code to the final users. Here is where the code will be deployed in production. This is also made available automatically through a set of rules pre-defined by the development team.
References
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Continuous Integration vs. Continuous Delivery vs. Continuous Deployment by Brent Laster. Book on O’Reilly.