The code used to run the blockchain network and specifications for how the system should work. This includes how the blockchain is updated (consensus mechanism), the network is upgraded, and how any cryptocurrency comes into play. Depending on the blockchain, it's maintained by a developer community or company and is often open-source. See the software for Bitcoin and Ethereum‍‍

A group of nodes (computers) running the blockchain's software and connected over the internet. Each node stores a copy of the blockchain and collectively the network maintains and updates the blockchain per the protocol. Any user that wants to interact with the blockchain must do so through a node. Generally anyone can run a node given the right equipment and the more nodes the more resilient the network becomes

Essentially a shared database (ledger) that can serve many different use cases. In Bitcoin it serves to track accounts and balances, but for Ethereum it serves a broader purpose as the backbone of the Ethereum Virtual Machine

The native asset of the blockchain system as defined by the protocol/software, but what it means/represents varies dramatically...
- the application/purpose (currencies like Bitcoin)
- ownership (like stock in a company)
- currency to use the blockchain system (Uber credits for the Uber app)
- all of the above

Blockchain technology allows for a shared database of information without the need for trusting any one particular party to verify accuracy. It's foundational in the same way as the internet allows for communications of data between various parties

Bitcoin was the first mainstream application of blockchain and is specifically focused on storing and transferring value. Similarly, email was one of the earliest applications of the internet, focused on communicating messages

Ethereum, another early application of blockchain, has a broader focus of being a platform for apps to be built and run on. Similarly, the world wide web, another early and key application of the internet, enabled websites and much of the internet services we use today

Web3 is the growing industry resulting from blockchain technology. Ideally it's a re-building of many of tech services we use today, but with a core focus of decentralization (using blockchain). It's comparable to the internet tech industry made of companies like Google, Facebook, PayPal, Amazon, and AirBnB that made internet technology the key component of how they re-imagined their markets‍‍

Generally, to use internet company services you pay in dollars (or other currency) and to own part of an internet company you buy the company's shares. Crypto takes those two roles for blockchain companies/services. Making it harder to explain is that it depends on the rules setup by each blockchain...