Echo

This command print as output its entire argument on the command-line.
It could be used with variables, like in the following example:

$ example:"this is an example"

$ echo $example

this is an example

Cat

This command prints the content of a certain file as an output on the command-line.

As example, we could imagine a simple text file in pages named "Hello", which contains the line "Hello World".
So, our command example will look like this:

$ pages Hello

$ cat Hello
Hello World

Grep

This one behaves very similarly to the previus one, but in this case it prints only the lines matching a certain pattern.
In this we could imagine a certain text file "animals", which contains a list of animal-names.

If we want to select only the animals whose name begins with the letter "d", we will write this:

$ grep d animals
Deer
Dog
Dolphin
...