SOA and SaaS: How do they differ?

Here is an interesting post that explains the essential difference between SOA and SaaS 

The simple answer is that:
Both SOA and SaaS are hot topics in IT as both deliver in very specific ways material reduction in costs of IT operations and deliver a more agile alignment between business needs and IT solutions. The manner in which those benefits are delivered is the key difference. And in many ways, to capture the majority of the cost reduction and agility benefits it is often best to incorporate both SOA and SaaS in your IT landscape. For complete story see here