Zipcar is established primarily in urban areas--users pay a yearly fee and can then rent cars by the hour for a fairly low price (usually between $7 and $15). Those cars are found throughout a given city instead of at a centralized location, and Zipcar's garage consists primarily of desirable cars like the Toyota Prius and Mini Cooper.
The service has found significant success among city-dwellers, college students, and others who have no need for a full-time car. Zipcar has expanded rapidly, acquiring competitors StreetCar and FlexCar as well as moving into new territory.
Though Zipcar has not yet shown a profit, that's largely due to its rapid expansion. This IPO proposal seems to be in that same line--Zipcar is trying to get its infrastructure in place, expand to over 100 markets around the world, and the profits will come later.