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 create and/or add input to a file:

samizdata# echo "This will overwrite whatever is in the file" > /root/testfile
samizdata# more /root/testfile
This will overwrite whatever is in the file
samizdata# echo "This will append to whatever is in the file" >> /root/testfile
samizdata# more /root/testfile
This will overwrite whatever is in the file
This will append to whatever is in the file

See redirection for more on using > and >> and other operands to move data to and from standard input, standard output, files, and programs.