The Fly has the ability to grant it's master flight. In the Clow Card's arc, The Fly sprouts wings at the head of Sakura's sealing wand so she can ride it like a witch's broomstick. After its transformation into a Sakura Card, it forms as wings on Sakura's back. Fly is a passive card with only mobile capabilites, and is Sakura's most used card. It's true form is a large (but can also become small as shown in the manga), white bird with a short beak, similar to a cocktiel's. It is under The Windy.
The Fly is the first card to appear in the anime after the seal on the Book of Clow is broken. Cerberus was teaching Sakura about her sealing wand and the Clow Cards when it shows up, screeching. Sakura has difficulty capturing it at first, but then weakens it with Windy, the only card in her possession at the time, and was able to seal it. In the manga, though, The Windy was ineffective in subduing it, since Fly came from The Windy's magic. Sakura then gives up for the night and goes back to bed. While asleep, she had a prophetic dream about the Fly's main body being wounded. On her second attempt to capture Fly, she realizes that it was injured, just like in her dream. She uses the The Jump (which she caught in the first chapter of the manga) to reach it up in a tree, and calms it down. Sakura realized that The Fly was not an agressive card, but was frightened due to it's injury, and it came to her willingly. From then on she used it often to scout, persue another card in mid-air, or escape.
The Fly is transformed into a Sakura Card in Episode 51. Eriol Hiiragizawa had enchanted a teddy bear to enlarge itself and terrorize Yue. To stop it, Sakura would have to use The Fly to reach it's head and The Sword to cut it off. At first this seemed impossible, as both cards used the sealing wand when released, but Sakura asked Fly to help her find a way to defeat the demonic bear. When she transformed The Fly, it sprouted wings from her back instead of her sealing wand, leaving her free to also transform The Sword, which still used made use of her sealing wand. After this, she was able to use Fly along with another card. There was a blooper during Episode 55 where Sakura transformed The Fly a second time. Though, this might also have been because she was in Wonderland.
In the second movie, Sakura and the Sealed Card, it is one of the last five cards Sakura has with her when she confronts The Nothing. It is the third one out of the five she used and used it to dodge one of The Nothing's attacks. Though, Nothing had no problem taking this card as well, leaving her with only The Shield and The Nameless Card.