To maintain a good information security, we need to maintain a CIA Triad. C – Confidentiality, I – Integrity, A – Availability.
When a sender sends some data/message/packet, it should only be read by receiver and not anyone else. Here receiver is a authorized user to read that data send by sender and others are unauthorized users. When a authorized user(receiver) reads the data/message send by sender, we can say that confidentiality is maintained. This is what ideally should happen, but many times attackers try to gain unauthorized access and read that data/message sent by sender. In such situations we say that confidentiality is lost.
When sender sends some data/message/packet, it should reach to receiver in that original form. Now what’s this original form, it means the data received by receiver must be in same state, with same meaning, with same data as it was sent by sender. This is what ideally should happen, but many times the attacker try to modify that sender data, attackers try to induce some error bits into the data. because of this the meaning of message/data changes completely. In worst situations the data is so corrupted that it becomes meaningless. In such situations we say that integrity of data is lost.
The resources should always be made available for a valid/authorized user. Whenever the valid user wants to access resources like memory units, software, they should be available. But attacks like Denial of service(DoS), the attacker try to make the servers unresponsive to the valid users resource requests, hence availability is lost.