This are the main characteristics of a villanelle:

•  6 stanzas and ends with a quatrain 
•   3 lines for each stanza, except for the last, which is 4 lines
•   Rhyme scheme:
  ABA
  ABA
  ABA
  ABA
  ABA
  ABAA

•  3 rhymes
•  The first line is repeated in line six, twelve and eighteen
•  The third line is repeated in line nine, fifteen and nineteen
•  Rhyming sounds: 2 sounds
•  Regular syllables in each verse

This is a fixed structure, which means that a villanelle has to be written exactly as described above. Consequently, you are restricted in using words. Therefore you must choose your words very carefully and make sure you choose words which have rhymes that are relevant to what you want to write about

This is an example of a villanelle (you should be able to find out what it is about):

We are the change that we seek; yes, we can
He said to a cheering throng
Proudly presented by a black man

A new era in America he began
It is not likely that something can go wrong
We are the change that we seek; yes, we can

For the health care problem he had a plan
The Americans have been waiting so long
Proudly presented by a black man

The man supports the people in Japan
To be sure he is very strong
We are the change that we seek; yes, we can

From postman, to deskman, to newsman,
the plans that satisfy lifelong,
are proudly presented by a black man

Think about all the people he outran
To the perfect men he may belong
We are the change that we seek; yes, we can
Proudly presented by a black man