User personas originated with Cooper
, in an attempt to design for real people.
Run a search on Kim Goodwin. She's a great resource and uses plain language. Here is an interview by UIE - Personas and Goal-Directed Design
There have been several variations of creating and implementing user personas. Obviously, the best ones come from some real data on users based on surveys with demographics, but this info is hard to obtain and never cheap.
Digging through usability studies is another way (how I do it), and looking for behaviorial studies, human factors, eye tracking...anything that offers insight into how people interact with web sites.
FutureNow, specifically the Eisenberg brothers and their team, have devised 4 personalities they use to guide design. My favorite is the "Methodical", which is what I use the most in my usability reviews. This person doesn't take any crap and has no patience. If you can sell to them or offer them an effortless experience, they return, make referrals, and convert.
One technique is the "Storytelling" technique. This is where you create a character. I compare this to acting in a play or acting class and you've just been asked to "Play the part of the multi-tasking divorced mother of 6 who has about 3 seconds to search for online courses." You describe her state of mind, emotional state, environment she's working in, type of computer, OS, browser, resolution, how many times the phone rang while she was searching with a baby on her knee and another kid is asking for math homework help..."
It can be a valuable tool in understanding that most people don't sit in a cube totally focused on their PC with their headphones on.
It's when you apply this character to a design that it gets insightful. Distractions in the environment make it difficult to stay on task and if the navigation or click paths are complicated, this person has far less time to deal with it.
Accessibility is another huge area. A user persona with ADHD is a potential user. So yes, there are limitless user personas that can be created.
This is why I like personalities approach by FutureNow. Global sites also need to consider how someone from another country may interact with their site. If no actual people are available or user testing is out of the budget, coming up with situations and characters helps flush out design issues.
I use user personas in my work as a teaching tool because often this is the first time a company has ever considered that people use their site differently than they do.