CRYPTPGRAPHY

Confidentiality consists of privacy, in this way sniffing is not possible, so data is only visible for people that have the rights to see it.
Cryptography consists of writing words in a different way, and its a way to keep confidentiality.
It is the set of techniques that allows you to encrypt the data.

Encrypt and Decrypt:

Encryption consists of modifying the data using cryptography, so other people can't understand it.
Decryption consists of discovering the way that the data has been encrypt of.

Algorithm: steps or patrons that you can follow to "cifrar" a word or a phrase.

The relation between an algorithm and cryptography is that you use algorithm to carry on cryptography. For example, there are some algorithms that move a letter one place forward or backward, other way to encript something is using numbers instead of letters and then, using algorithms, you mix those numbers and after that, turns them again into words. this way most people wouldn't be ablo to discover the data, but they can find a way to decrypt it too...

A key is the number you use in order to encrypt something using algorithms.

Here I leave you a link to enter in a web that shows a game to encrypt and decrypt data. Using the "cifrado cesar", you use a key (must be a number) and depending of that key you change a letter for another. for example, if you want to change an "a" using key 4, you must write an E, because you must move the "a" in the alphabet as places as the key indicates.
http://www.cryptoclub.org/