Personas

Personas are an excellent tool for getting a full understanding of your user group(s); encouraging empathy; and, in general, "getting everyone on the same page" regarding the user experience. Common goals, wants and pain points are generally presented along with a brief profile write-up for each segment group.  

Types of personas can vary. They can be a lesson in creative writing crafted through input with key stakeholders and basic information known about users. They can be created based on marketing metrics and demographics. Or (and this is my favorite) they can be created based on numerous interviews with actual users whose input helps bring to life a fleshed out personality. For my money, the ideal persona is a combination of all of the above.