Echo

echo repeats whatever has been given to it. From the manpage:

    The echo utility writes any specified operands, separated by single blank
    (` ') characters and followed by a newline (`\n') character, to the stan-
    dard output.

The simplest way to explain it is to see it in action:

samizdata# echo "hello!"
hello!

It can also be used to add input to a file:

samizdata# echo "This is a test file" > /root/testfile
samizdata# more /root/testfile
This is a test file
samizdata#