Floo powder

Floo powder may be purchased at Wiseacre's Wizarding Equipment for the low low price of 2 Sickles per scoop.

Uses
Floo powder will allow you to travel to some destinations outside of Hogwarts. You won't want to disappear too long lest you be discovered and lose some points for your house. One scoop will let you explore for up to an hour before having to return to school grounds.