TCP/IP is a protocol used to to communicate over the network. TCP/IP architecture consist of 4 layers basically :
Top => Application Layer
Second => Transport Layer
Third => Internet Layer
Fourth => Network Interface Layer
How these layers work ?
Application layer : http,ftp,telnet and smtp are the part of this layer. Ultimately application layer is used to services provided by other three layer. Like sending and receiving the data through HTTP is a great example of application layer. FTP application which helps the user to send any file on any another system on the network.(But internally application layer requires the support of other three layer).
Transport Layer : is a Host-to-Host type connection oriented layer. Transport layer were there before HTTP,FTP and other application layer.
As Transport layer is the main basic layer which actually sends the data in the form of packets. The sequencing of data packets and recovery from error is all done through this only.
Internet Layer : The task of packaging the data(in the form of packets),addressing(where to send) and routing them to a specific location is done through Internet layer. IP address,Physical addresses to do so.
Network Interface Layer : Ethernet, LAN Card. At the system level when the data is received(packets actually) the task of placing it properly on NIC is done by this layer.
Always it doesn't include any task like sequencing,communication through session,are all in Transport layer.
Now let's understand all 4 in simple way.
Suppose you visit : http://www.google.com
Now, you have sent the request using HTTP protocol,which is implemented at application level not at hardware or physical level.
Here the application with http(browser) will have one to one connection with the Google server application which is actually receiving this request and respond to you on your browser.
But it's the not the application layer working, the other three backbone support application to do it's work something like this.
In case of sending request to Google.com; Transport layer will manage the task of reliable connection,continuous connection,sequence them and find the error.
Just like that Internet layer will actually do the task of sending packets into exact IP address with proper fragmentation.
When the computer receives the data packets,but before that it receives in the form of bits, the movements of these bits inside your computer through NIC and outside is actually done by NIL.