There are many forms to any particular word in the English language, that is, verb, noun, adjective and adverb. With minor changes (additions and/or deletions of letters) words can take up different forms.
1. notice - notice (same spelling, but 2 meanings)
The verb "notice" means to take note of, observe
The noun "notice" means a board, paper containing information or instructions.
I did not notice my mom standing right beside me.
There was a notice on the board saying we should pay the fees today.
2. act-action-active-actively
Verb: I'm going to act in a play in my school.
Noun: Such behaviour will give rise to strict action.
Adjective: He is an active member of the student union.
Adverb: Ravi participates in charities actively.
3. judge - judgement:
Verb: Please don't judge a book by its cover.
Noun: The judgement is due today.
4. Drive
Verb: I'm going to drive to Bangalore on Monday morning.
Noun: The drive to Bangalore was very scenic.  
5. Challenge
Verb: I challenged my friend to complete the work at my speed.
Noun: My friends accepted my challenge.
AdjectiveThis is a challenging exercise!
It is intriguing to know that there are hundreds of words that can be used as a noun, verbs, adjectives (sometimes by adding/deleting letters) in the English language.
List of words generally used in many forms can be found here.