Q:

What is the difference in the usage between "reservieren" and "buchen"? 

What is the difference between reservieren and buchen?  Don't they both mean to book? 

Ich reserviere eine Reise. 
  Ich buche eine Reise. 

A:

Buchen often includes a higher grade of commitment, maybe even already paying for it.  If you buchen something, it normally is yours - if you reservieren it, you often only prebooked it.  Sometimes you then need to get it in a certain time or you loose your Reservierung. 
Reservieren is very often used in context of seats (like in theaters or trains) or when you want someone to set something aside for you (e.g. in stores).  Buchen is often used for holidays, including flights and rooms - or if you engage someone (like a singer or a DJ for a party). 
Buchen normally includes some kind of contract and often includes a penalty fee if you step back, Reservieren normally doesn't. 
So in your example, I would say that Ich reserviere eine Reise is a bit unusual.  Maybe you would say Ich reserviere mir die Zeit für eine Reise, but if you get your tickets and your rooms, you probably would say Ich buche eine Reise. 

A:

The difference is about the same as in English. 
Etwas reservieren means that you express your interest in it, blocking it for other people who are also interested.  Such a Reservierung most of the time is only valid for a certain period of time. 
Etwas buchen means that you make a definitive statement about your interest.  Most of the time this is a binding statement for both parties (the person offering the service and the person who books the service), which you can only withdraw paying some kind of fee. 
An example would be: You're interested in a holiday apartement, but you have to ask your spouse first, so you place eine Reservierung, talk to your spouse and after that buchst du the apartement.