User Experience and Interaction Design both has come a long way. Now a days we can say it is the most important thing in web-design. Interaction Design bridges the gap between man and machine.
User experience, most often abbreviated UX, but sometimes UE, is a term used to describe the overarching experience a person has as a result of their interactions with a particular product or service, its delivery, and related artifacts, according to their design.
On the other hand Interaction design (IxD) is the study of devices with which a user can interact, in particular computer users. The practice typically centers around “embedding information technology into the ambient social complexities of the physical world.”
Definitions from wikipedia.org
Here is a great blog which talks about UX and IxD.
Please have a look.