Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A protocol is a set of rules that governs communication between devices: the format of the data, how it is addressed, how errors are detected and what happens when they are. Without agreed protocols, devices from different manufacturers could not understand each other. TCP/IP is a protocol suite arranged as a stack of four layers: Application, Transport, Internet and Link. Each layer has its own job, uses the services of the layer below and provides a service to the layer above. On the way down the stack each layer adds a header (encapsulation); on the way up the receiver's layers remove them.
"A protocol lets computers communicate" is too vague for the mark. Say it is a set of rules and say why rules are needed: so that devices from different manufacturers or platforms interpret the data in the same way.
Do not mix up the addresses. Port numbers belong to the Transport layer, IP addresses to the Internet layer and MAC addresses to the Link layer.
Sequence numbers are added by the Transport layer, not by IP, and it is TCP at the receiver that reorders the data and requests lost segments.
HTTP transfers web pages and their resources between a web server and a browser (HTTPS is HTTP made secure by encryption). FTP transfers files between a client and a server. Email uses three protocols: SMTP pushes an email from the sender's client to their mail server and on to the recipient's mail server; POP3 and IMAP let the recipient pull it from their server. POP3 downloads the email (usually deleting the server copy); IMAP keeps it on the server so every device stays in sync. BitTorrent is a peer-to-peer protocol: a file is split into pieces and each peer in a swarm downloads different pieces from many other peers at once while uploading the pieces it already has.
HTTP is not "the internet" and not "how web pages are displayed". It is the set of rules for requesting and transferring web resources between a client and a web server; the browser renders the page.
POP3 and IMAP are not used to send email. A common lost mark is writing "IMAP sends the email to the recipient": the recipient's client retrieves it.
The tracker does not store or send the file. It only coordinates the swarm by telling peers about each other; the pieces travel directly between peers.
In circuit switching a dedicated path (circuit) between sender and receiver is set up before any data is sent; all the data follows that path, in order, and the bandwidth is reserved until the circuit is released. In packet switching the message is split into packets, each with a header holding the source and destination IP addresses and a sequence number. Each packet is routed independently: every router reads the destination address and uses its routing table to forward the packet to the next hop, so packets may take different routes and arrive out of order. The receiver reassembles them using the sequence numbers. Links are shared by everyone, so packet switching uses the network far more efficiently.
"Circuit switching is faster" is not enough. Say why: once set up, the bandwidth is dedicated, so the data arrives at a constant rate, in order, without queuing at routers.
A router does not read the whole message, and it does not reassemble the packets. It looks only at the destination IP address of each packet; reassembly happens at the destination computer.
Modern internet phone calls (VoIP) use packet switching, not circuit switching. They cope with variable delay by buffering, and quality drops when the network is congested.