Here's a useful video on the topic:
For example:
'e' being transformed to 'E'
$ echo "Hello" | tr "e" "E"
HEllo
Spaces being transformed to hyphens
$ echo "Hello how are you" | tr " " '-'
Hello-how-are-you
Digits (numerals) being deleted
$ echo "Hello how are you 1234" | tr -d [0-9]
Hello how are you
This tutorial shows various ways that the tr command may be used for transforming or translating characters or character classes. It can also be used to delete, complement, remove or squeeze characters.