Mahou Shoujo: a genre in which a girl is given powers
Traditional Examples: Cardcaptor Sakura, Futari wa Pretty Cure, Onegai My Melody, Pretear
Other Examples: Jubei-chan Ninja Girl, Getsumen to Heiki Mina, Mai-Hime, Pretty Sammy, Mao-chan
Common themes in Mahou Shoujo series are transformation scenes, frilly outfits, animal/creature sidekicks and catch phrases.
Transformation scenes are notorious for having the girl in a semi-nude form. The clothes and boots or accessories materialize out of light, magically.
Some animal sidekicks include Kero (from Cardcaptors), Ran, Miki, Su, Diamond (from Shugo Chara!), and Ryo-Ohki (from the Pretty Sammy series.)
The most popular catch phrase for magical girls is "I will punish you" or "punish evil!"
The Old: Sailor Moon, Cutey Honey, Himitsu no Akko-chan, Moldiver, Floral Magician Mary Bell
The New: Tweeny Witches, Sugar Sugar Rune, Birdy the Mighty Decode, Happiness!
Mahou Shounen: a genre in which a boy is given powers
Examples: DNAngel, Lagoon Engine, Naruto, Death Note