The most common type are the small silk bags you see in my photo, having inside a prayer written on paper or wood. Mamori 「守り」 means "protection", but depending on the prayer, they are intended for good luck, good health, traffic safety, business prosperity, household happiness, entrance exams, pregnancy protection, love, etc.
An interesting fact is that an omamori must not be opened: not only that the opening will be an act of disrespect, but it is said that the charm will loose its powers or even bring bad luck...
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 18mm
Aperture: F/5.6
Shutter Speed: 1/125s
ISO Sensitivity: ISO 200