With Google's successful demonstration of "quantum supremacy" in 2019, the research of superconducting quantum computing is attracting more and more attention around the globe. Superconducting quantum bits (qubits) are macroscopic devices with fundamental quantum properties such as energy-level quantization, quantum-state superposition, and quantum-state entanglement. They provide an excellent platform for studies in many fields such as quantum physics, atomic physics, quantum optics, quantum chemistry, quantum simulation, quantum computing, and so on. In this article, we will discuss the basic principles and structures of the superconducting phase-, charge-, transmon-, and flux-type qubits, as well as their device design and fabrication process. We will also present a brief introduction to the rich research areas based upon the superconducting qubit architecture.