Definition and purpose
Magical Girls are usually ordinary girls who are granted super powers in order to save the Earth or protect something. They are usually forced into doing this, either by their own mistakes for which they must make up or by important persons who support their choice by saying the Girls are pure-hearted and/or have the necessary powers.

Powers and power-ups Powers are of 2 types: originated (the Girl was born with them) or obtained (she obtains them after training/learning). Originated powers can manifest themselves during the Girl's life (before she finds out 'officially' about them) and increase after she finds out they're 'real' and not just some 'gift'. The obtained powers aren't so common and are mainly magical powers (spells, charms etc.) or physical abilities (super speed, agility etc.).
Frequently, the Magical Girls have to protect an important item from a being who wants to use it to rule the world. But when the Enemy obtains the item upon defeating the Magical Girls, the hidden powers come into play. These are used only in case of extreme emergency and are summoned by complicated rituals or the power of Love, after meditation or immediately. Sometimes the item itself plays an important role.
Power-ups are a must-have in long anime series, since people tend to be bored if a Magical Girl has only one attack/transformation phase in 50 episodes.

Magical items and attacks A show with Magical Girls cannot exist without a magical item. From staffs to body parts, they are used as a mode to direct the possessor's energy. Items can be used for both defense (in the beginning) and attack (when the Magical Girl isn't afraid of her Enemy any more). Also, a Magical Girl can rarely transform without a particular item she keeps with her all the time, like a locket, hair clip or ring.
When such an item is broken or otherwise out of order, it is a sign of an incoming power-up. This can happen at random times and/or in a decisive battle with the strongest Enemy.

Allies and Enemies
In the beginning, Magical Girls have a Mentor who guides them through their first battles and teaches them how to act and fight, as well as provides them with data about the enemy. Mentors often disappear after a few episodes, either dying or going on important missions.
Here's where the Assistants (or Allies) enter the scene. Magical Girls can be aided by creatures (very often), extraterrestrial beings, spirits, humans who don't possess magical powers or other Magical Girls (teams are a frequent and popular subject of anime series).

Secrets and costumes
Of course, the Magical Girls have to keep their lives secret from others (the police, their families and friends). This can lead to arguments and people trying to find out the secret. Costumes are for protecting the Girls from the 'good guys' who would turn bad upon finding it out.
More information: Magical Girls on Wikipedia | The shoujo genre

Sailor Moon (Bishoujo Senshi Sailor Moon)
A classic and a childhood source of inspiration for many, Sailor Moon is the story of Senshi (Soldiers) who protect the Earth from invaders and evil-doers. In the beginning there are only 5 Senshi but more join the team as the story progresses, fighting powerful villains and discovering their past.
Considered very cliche by many, this anime is still very popular and spans 200 episodes (in 5 seasons), 3 movies and countless merchandise items with worldwide success. More info...
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Tokyo Mew Mew (Mew Mew Power)
5 girls are infused with the DNA of endangered animals which gives them the ability to transform into Mew Mews, beings with special abilities like super speed and agility, not to mention super powers. The story focuses on Ichigo, Mint, Lettuce, Pudding and Zakuro, the unwilling heroes who must now save the Earth from the invading aliens who plan to conquer it. Along the way they form friendships and discover new enemies and allies.
Running for 52 episodes, TMM aired from 2002 to 2003. 4Kids dubbed 26 episodes but the other 26 aren't available. More info...
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Card Captor Sakura (Cardcaptors/Cardcaptor Sakura)
10-year-old Sakura Kinomoto, a 4th grader, opens a mysterious book in her father's study and accidentally lets loose the magical Clow Cards. Because she was able to release the cards, Sakura is revealed to have special powers and her duty now is to get them back, aided by Cerberus (Kerberos/Kero) and Syaoran Li.
CCS aired from 1998 to 2000, spanning 70 TV episodes (in 3 seasons), 2 movies and and several specials. This CLAMP title is known as one of the 'most' shoujo titles, due to the flowers, bubbles or sparkles present on almost every page of the manga. More info...
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Puni Puni Poemi (Puni Puni Poemy)
PPP is the humorous story of Poemi Watanabe, a hyperactive school girl whose big dream is to become a voice actress. However, her plans are in danger when aliens attack the Earth. Receiving the power of a dead fish which acts as a magic wand, she battles the aliens with the aid of the Aasu sisters, who seem to have 'magical' powers.
In its essence, PPP is a parody of the shoujo genre (especially Poemi's transformation sequence which resembles Sailor Moon's). This spin-off spans 2 OVA episodes and aired in 2004. More info...
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Rozen Maiden
Jun Sakurada is Junior High School student who refuses to go to school after a painful event and spends his day ordering things online. One day he comes across a mysterious doll which comes to life and turns him into her medium. The doll, whose name is Shinku, is the 5th Rozen Maiden doll, competing against Suigintou, the first Doll, in the 'Alice Game'.
The Rozen Maiden anime is divided into 2 seasons and 3 OVA episodes. There are also 3 games. More info...
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