Networked systems, especially the internet, were basically built out of Cold War paranoia. The idea was to create a communication system that could survive a nuclear attack. Instead of routing everything through one central point, the network was distributed, so even if part of it got blown up, the rest would keep running.
Packet switching is the core idea that makes all of this work. Instead of sending data as one continuous stream, it breaks files into small packets, ships them across different routes, and reassembles them at the destination. That makes the whole thing way more reliable and efficient than older systems like telephone lines, which needed a dedicated connection the entire time.
TCP/IP is what allows different computers to actually talk to each other using a shared standard. Data gets wrapped in something called a datagram so it can travel across networks and land at the right place. Every device gets a unique IP address, and DNS exists because nobody wants to memorize strings of numbers just to visit a website.
It's also worth understanding that the internet and the World Wide Web are not the same thing, even though people use them interchangeably. The internet is the infrastructure. The Web is a system built on top of it for accessing and linking information. ARPANET started out as a way for researchers to share computing power, but email kind of hijacked everything and turned the whole network into a communication platform instead.
Commercialization is what really changed the shape of things. Web browsers made the internet accessible to normal people, which caused an explosion in websites and online businesses. That growth also inflated the dotcom bubble, where speculation drove valuations to absurd heights before the whole thing crashed back down.