An interaction designer defines the interactive processes of applications and products in response to human behavior.

What does that mean?

Interaction designers make things such as website services, software applications and interactive physical products easier to use by the general public or a specific set of users.

Do interaction designers make the product or service faster?

No, thats what coders do although interaction designers can make the site goal or tasks more quicker to execute by the user (rather than the computer) allowing the user to do more or view more information.

Do interaction designer make things look nicer?

No, thats what visual designers/graphics artists do.

Do interaction designers make information easier to find?

No, thats what SEO specialists do before users reach a website, thats what marketing specialists do to promote knowledge of a product or service before a user finds out a product or service exists and thats what information architects do once users are on a website.

Do interaction designers make content easier to understand?

No, thats what information designers do making sentences easier to understand by better wording, paragraphing and pagination.

Then what do interaction designers do or do to aid the above?

Content Publishing:

If you have a very complex work flow for entering content into a site with multiple requirements and options, an interaction designer can create the most efficent workflow that is not mentally taxing.

Content Finding:

An interaction designer can select currently existing UI patterns (drop down lists, drop down menus, vertical tabs, etc..) to make content finding easier or in the event of a technology breakthrough (the next AJAX or Javascript feature) interaction designers can develop new UI patterns. The key difference between an interaction designer and an information architect in respect to content finding is that interaction designers can develop new UI patterns that don't already exist universally (or exist already but not within a medium such as a specific development platform.)

Goal or Task execution:

Interaction designers can make purchase flows faster and less cumbersome. Similar to content publishing, task execution such as document sharing within a specific network of connected users, document editing by users can be made more intuitive by an interaction designer. As more options are added and more criteria is required before a piece of content can be created accessed, the more the importance of assigning the work to a fully capable interaction designer rather than a similar practitioner of related disciplines.

How does the discipline of interaction design aid in the end-user experience and the overall perception of the product and its parent brand?

By having the most intuitive (least cumbersome) method of executing tasks, the rate of abandonment  of a product or service is lowered (due to easier work/user/task flows) and the overall perception of production quality and attention to detail is increased.