Technical Writers and Instructional Designers are two different people in the same boat. Its like providing training to the user as well as providing help to the user. So, while Instructional Designers design the content for a particular training before using the application whereas Technical Writers produce help requied while using the software application. But that doesnt mean technical writers cant produce training materials or vice versa. Also, IDs are generaly required in the elearning companies and TWs and required by both elearning and product companies. But the trend is changing these days. You will find both of these in all the companies.

~Anuj Kapoor

Instructional design is transforming of knowledge. Many companies require instructional designers to prepare quality documents which require instructions. Instructional designing involves analyzing the learning needs and delivering the same.