Blockchain (or distributed ledger) technology was introduced in 2008, when the famous Bitcoin cryptocurrency was initiated. Blockchain has been undergoing rapid growth in both academia and industry. Today, it is no exaggeration to say that blockchain has become a new, independent research topic—not a subtopic subsumed within cryptocurrencies. From a technical perspective, blockchain technology is based on various fundamental computing technologies, such as advanced cryptography, distributed data storage, peer-to-peer networking, and distributed consensus protocols. Generally, blockchain technology involves creating a shared, distributed ledger: that ledger can offer great flexibility and potential in resolving many important challenges in a complex computing context that involves multiple parties. Examples of such challenges include achieving mutual trust, privacy protection, and data consistency in large-scale business scenarios. Many business applications have already covered a broad range of industrial services, such as those related to finance, governance, medicine, and city construction. Blockchain technology is becoming increasingly adopted and applied; however, the current design of blockchain is practically far from sufficient—especially when dealing with critical domain challenges. Specifically, the key limitations of blockchain mainly derive from poor system scalability, weak resilience to external security attacks, and lack of computing interfaces for regulatory processes. Conversely, it is the very shortcomings of blockchain technology that motivate research efforts into many related technologies. Based on conventional blockchain design, new functional extensions and cryptography technical optimizations have been continuously proposed by researchers and practitioners: the aim is to make blockchain technology more practically applicable and meet various demands of different business users. In this overview paper, using the latest findings from both academic and industrial research, we systematically present the general architecture of blockchain technology with its five functional layers. The five-layered architecture comprises the following: a data layer; a network layer; a distributed consensus layer; a smart contract layer; and an application layer. We also provide a technical description of key theories and important techniques related to each functional layer. From the proposed general architecture of blockchain technology, we offer an in-depth explanation of its core technical extensions with respect to the following: blockchain integration with existing computer network techniques; the blockchain framework itself and important modules; and underlying critical cryptography techniques. Further, we discuss potential contributions that these promising technical extensions could provide with respect to reshaping and optimizing blockchain technology. Finally, following current developments with blockchain technology and its existing mainstream applications, the general views about future challenges and important directions for this technology is to facilitate future follow-up research.