Internet is network of networks. Computers running programs (and other Internet Devices) can connect and communicate. It's like graph (nodes and connections - or dots and lines that connect them).
Network Edge consists of devices that we are most familiar with (computers, PDAs, cellphones, tablets and other MIDs [Mobile Internet Devices]) that we use on daily basis. They are connected together into networks that are also interconnected (via Network Core - hierarchy of ISPs [Internet Service Providers] ).
Software running on Internet Devices communicate using protocols. Protocols organize the way software interacts, order and structure of messages exchanged.
Protocols are separated into Layers, each Layer providing certain services to the infrastructure.
Five-layer Internet protocol stack consists of:
Application Layer (communication/interaction between applications).
Transport Layer (makes sure that right program running on device receives right information, and provides additional services such as 'guaranteed' delivery of messages, information flow speed control, and other internal technical services such as congestion control and breaking application-layer messages into segments. Depending on transport layer protocol (TCP or UDP) some services may be provided, some might be not).
Network Layer (responsible for moving information packets from source device to destination device [possibly with more than one Internet Device on the path to traverse through]. The internet network layer includes IP Protocol, which defines the how transmitted information is categorized, and how systems react on this information. The Internet's network layer also contains routing protocols that determine routes that information packets [in this layer named datagrams] between sources and destinations.
Link Layer (responsible for moving packets [named 'frames' in this layer] from one Internet Device [router is also Internet Device] to another in one hop).
Physical Layer (while the job of the link layer is to move entire frames from one network element to an adjacjent network element, the job of physical layer is to move the individual bits [of information] within the frame from one node to next. The protocols in this layer depend on actual transmission medium of the link [for example, twisted-pair copper wire, fiber optics]).