ImageMagick is a robust collection of tools and libraries to
read, write, and manipulate an image in many image formats
(over 68 major formats) including popular formats like TIFF,
JPEG, PNG, PDF, PhotoCD, and GIF. With ImageMagick you can
create images dynamically, making it suitable for Web
applications. You can also resize, rotate, sharpen, color
reduce, or add special effects to an image and save your
completed work in the same or differing image format.
The above text is from the ImageMagick Website
therefore not everything does apply to this extension at
the moment, but it will some time in the future.
Hopefully. And the link above is also a great source, if
you don't understand everything here.
imagick went through a major upgrade with version 0.9 and is not backwards
compatible to older versions. You should look at the examples, which come
with the package, for some documentation about the new API.