If you were asking about a transformer used in electricity, a transformer is basically a device helps convert high voltage electricity to low voltage electricity. The electricity in the power lines outside - is much stronger than we need in our house or school.
Here's an article I found on the on-line resource Britannica School Edition (Middle School):
http://school.eb.com/comptons/article-9277410?query=transformer&ct=
You can find a link to Britannica on the VLC here:
http://vlc.ucdsb.ca/az.php?a=b
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If you meant the toy Transformers - they are cool cars or other vehicles that turn into (transform) fantastic robots with special powers and gadgets!
http://www.hasbro.com/transformers/en_CA/