Networking has two competing standard models for describing how data actually travels from one device to another, and only one of them is what the real internet is built on. The OSI (Open Systems Interconnection) model divides network communication into seven precisely defined conceptual layers, each handling a distinct responsibility. TCP/IP, the model the actual internet runs on, groups those same broad responsibilities into a leaner four layers, developed independently, earlier, and for a different purpose than OSI's more formal theoretical framework.
OSI's seven layers were designed as a complete theoretical reference
The OSI model, developed through international standards work in the 1980s, defines seven layers running from the physical transmission of raw bits at the bottom, up through data link, network, transport, session, and presentation layers, to the application layer at the top, where the actual content a user cares about — a webpage, an email — lives. Each layer is defined to handle one specific, cleanly separated responsibility, and to communicate only with the layers immediately above and below it, an elegant, comprehensive conceptual structure intended to serve as a universal reference model for describing any networked system's communication, regardless of the underlying technology.
TCP/IP came first in practice, and won by simply being what was built
TCP/IP, developed earlier and separately from OSI as part of the practical engineering work building the early internet, organises the same broad set of responsibilities into four layers instead of seven — link, internet, transport, and application — a leaner grouping that maps roughly, but not precisely, onto OSI's more granular seven-layer structure. Crucially, TCP/IP wasn't designed as a competing theoretical model in the same sense OSI was; it was the actual working protocol suite that real, functioning networks were already built on by the time OSI's more formal specification was fully developed, and that early practical head start is a substantial part of why TCP/IP, not OSI, became the protocol suite the real internet actually runs on today. The OSI model still survives widely as a teaching and conceptual reference tool, useful for discussing and reasoning about network architecture in a standardised vocabulary, even though the real infrastructure moving data across the internet runs on TCP/IP's leaner four-layer structure, not OSI's seven.
What we're still unsure about
The structure and historical development of both the OSI and TCP/IP models, and the fact that TCP/IP is what the operational internet actually runs on, are well documented and uncontroversial within networking. What's less a matter of settled fact and more a matter of ongoing pedagogical convention is exactly how directly OSI's seven layers should be mapped onto TCP/IP's four in teaching contexts, since the mapping isn't perfectly clean — some OSI layers correspond fairly directly to a TCP/IP layer, while others are grouped together or handled somewhat differently, and different textbooks and courses present slightly different versions of exactly how the two models line up against each other.
This sits inside OSI & TCP/IP Models, one of seven topics in Networking, one of seven domains in Computer Science, one of seventeen subjects the app can quiz you on.