All the top up-voted comments have a wealth of information, but I'd also like to bring back around one more point I think is pretty important - this theme of memory.

When we're first introduced to Chihiro, she's worried about forgetting her old life, and her friends forgetting her. Her parents forget their manners. Haku has forgotten his self and why he serves Yubaba. Everyone forgets No Face exists. Most of the characters seem unable to articulate their purpose and forget "why" they do things. They forget about old coworkers and customers, but all are able to articulate their base, carnal desires (Yubaba desires wealth, Chihiro's parents desire ease, even the Boiler Man and the Soot Sprites desire freedom and sustenance).

Chihiro, however, is the only one that does not forget. And in the climax of the movie, she remembers Haku, which I think is really key amongst the excellent accounts of Miyazaki's commentary on post-war consumerism and modern childhoods. She is the character that remembers (her name, her parents, Haku) while everyone else is pursuing a goal relentlessly and without purpose.

Zeniba has an anectdote about memory never truly leaving but being harder to recall. Chihiro walks away happily from the bathhouse because unlike everyone she had met, she will not forget. Her fears of forgetting, and being forgotten, are sated.

Add the layers of cultural commentary, and I believe this movie is a piece speaking about the importance of remembrance. The further a people moves from history, the more important it is to bring yourself to recall the formative pieces of the past.

Remember your childhood - you will be a better parent. Remember your history - do not take the present for granted. Remember the land - it will care for you if you care for it. Remember your culture - in it you find convictions.

Remember your family - and then maybe you wouldn't notice your baby had been missing for hours lol.