There's a great thread over on the IxDA mailing list discussing "As an interaction designer, how do you explain your work in social situations?" Here are my favourite answers.
"I design software and websites that people don't hate." - Michael Micheletti
"I figure out what a web site or application needs to do, and why, and then design how people will perform those tasks." - Robert Hoekman Jr
"I make the web easier to use... one page at a time." - Tori Breitling
"Remember the last time you were yelling at your <insert device here>? Interaction Designers are being brought in to make sure that doesn't happen again." - Shaun Bergmann
"I make computers more human so that humans can act less like computers." - Michael Williamson
"I help make web sites and applications easy to use, and look good." - Hugh Griffith
"I make sure the computer works to make your life easier, instead of requiring you to make your work match the way the computer wants things." - Katie Albers
"When it comes to web or application design, I advocate on the user's behalf, rather than the client's." - Jen Randolph
"[I'm] like a normal architect, but instead of buildings, I plan websites." - Jane Mountain
"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." - Angel Marquez
"You know that person who figures out what goes where on the screen on sites like Amazon so that you can figure out what to do? That would be me." - Julie Stanford
"I make technology easy and fun to use." - Dan Saffer
And finally...
"I don't get invited to parties." - Andy Polaine