How does uln2803 work??

hi I am making a project using stepper motors and uln 2803.. Can someone please explain me what is uln2803, why is it required and what does it do and how??
Thanks so much..

How about learning to use Google!

This is what PDF's are for =)
ULN2803 Datasheet pdf - OCTAL PERIPHERAL DRIVER ARRAYS - ON Semiconductor
It's all in there.

For simplicities sake, it takes a low input current and provides a much higher output current, micro controllers and most digital logic can't drive higher current devices.