Leisa Reichelt, a user experience consultant, sells her services and at the same time does a great job of succinctly explaining what a customer journey map is and why it should be important to you.
She also provides links to further reading, only some of which I've read and blogged previously. The new ones are worth a read and I'm unashamedly referencing them here :)
Bruce Temkin's paper on mapping the customer journey leads us through a 5 stage process to generating a map, provides some examples illustrating the variety of approaches used and then provides some advice on making the most of the map once it's created.
Service design tools have pulled together resources and activities to help facilitate a group of stakeholders working together to generate a customer journey map. Some of these look excellent - I'm definitely going to be trying out the game at the soonest opportunity.
And a couple of articles explaining what it's all about and why those of us involved in service design and delivery should be embracing the technique:
Using Customer Journey Maps to Improve Customer Experience – Harvard Business Review