In the third in the series, Dann introduces us to the tr command.
Here's a flavour:
The tr, or translate (aka: transliterate) command, substitutes one more characters for another set of characters or it will delete a specified set of characters. The tr command takes input from standard in and writes to standard out. This simple example of the tr command translates some numbers into a word:
echo "12234" |tr '1234' 'aple'
The entire article, including links to the videos can be found on his site: