There are a number of differences - some obvious, some not so obvious. They range all the way from surface-level practice (we cross ourselves right to left) to deep doctrinal divides.
Some one once said that the difference between East and West can be summed up in three words: Augustine, Anselm, and Aquinas. In a very real way the difference between East and West is more one of world view than of a point-by-point doctrinal comparison. We approach the subjects of Ancestral Sin, Soteriology, and the nature of the God/the Trinity in very different way so that even the things that seem similar can mean something else or have contrary implications.
If you really want to learn the differences, I would say there are three questions to start with that will help you to understand an Orthodox mindset.
1. Why is the Incarnation of Christ important and what is it's relation to the saving aspect of the Crucifixtion and the Resurrection?
2. What is Salvation and how does one go about aquiring it?
3. What is the nature of Divine Grace and what implications does that have with regard to the Sacraments?
I won't try to answer these here, each one is worthy of a volume (or two). I think if you wanted to really understand the differences between Catholic and Orthodox you could compare how each would answer the above.
