I am asked this question quite often, especially at things like jobs interviews and by my students. They are looking for a one sentence answer, which is quite difficult because the field encompasses so much. However, I think I have developed a one sentence answer that should give people an understanding about what ISD is. Keep in mind, this definition is very high level:
A systematic process for developing instruction, which improves learner achievement of a learning objective that is measured through assessment. To improve instruction, models, strategies, and theories are used to assess the current situation, create learning objectives, develop instruction that meets those objectives, and then to develop an instrument to assess those objectives.
Here is an image that depicts my definition of Instructional Design: http://www.raypastore.com/knowledge/ISD_process.jpg