To make sure everyone on the project has the same vision of who you're building for, create a single picture of "the user." Assumption personas are fast to create and easy to verify later.
Personas are a topic I've mentioned on this site before. If you want the full-on audiovisual extravaganza, head over to Lynda.com and watch my latest course, UX Design Techniques: Personas. This online video course contains more recent content than the site, and shows how personas fit in the product design and development lifecycle. The benefits of personas are a more focused development process and more likelihood of creating something that your target users actually want to work with.
Here's a video from the course. Check out the full course online.
Future episodes will cover the subsequent steps of the user-centered design process in more detail, including: using ideation techniques, storyboarding, designing and testing prototypes, and planning your development cycle with all this information. Lynda.com have already posted the Overview and Analyzing User Data courses that precede this one.
If you aren't already a Lynda.com member, or if your school or workplace doesn't have a membership, you can get 7 days of free unlimited access. - use it to check out this and all my other courses! If that isn't long enough, one month of access costs just $25.