What is the difference between Qiskit Terra, Aer, Ignis and Aqua?

Question

I am going through the Qiskit tutorials and they have an article on each of the above 4. however, As a beginner, I am not very clear. Could anyone share some insights on these?

Answer 1

Back in Qiskit 0.24 (around March 2021) Qiskit was organized in Elements, as KAJ226 points out. However, after 0.25, Aqua was broken up in domain specific packages and the Aqua deprecation process started. At the end of 2021, Ignis was also deprecated to be replaced by qiskit-experiments. The current Qiskit landscape looks roughly like this:

• Qiskit Terra (repo package), the core module that handles quantum circuit construction, circuit analysis and transformation, and general use algorithms, such as VQE.
• Qiskit Aer repo package), for quantum simulators.
• Qiskit Application packages, these are 4 modules, Machine Learning (repo package), Nature (repo package), Finance (repo package), and Optimization (repo package), they target specific domains and plug into the tools used by experts.
• Qiskit Dynamics (repo package) provides access to different numerical methods for solving differential equations
• Qiskit IBM Quantum Provider (current repo package, soon to be moved to repo), the module that enables Qiskit to access IBM quantum devices in the cloud.
• Qiskit Experiments (repo package), replacing Ignis (repo package), the module that contains all the required tools to implement error mitigation techniques.
• Qiskit Metal (repo package) is a framework for engineering and designing superconducting quantum devices.

This landscape is constantly changing and hard to follow. The best place to learn about past and future changes is this wiki page maintained by the Qiskit community team.