At the very initial application, Social Security will be obtaining your medical records. That's the foundation of your case, as your medical records show the extent and severity of your medical condition. They'll also be sending you forms to fill out about what kind of problems you have in your daily activities. That will also be used to evaluate your case. If your case needs to go to a hearing before an administrative law judge, they'll also be asking you questions about not only your medical problems, but what are your limitations you have as a result of your conditions. The judge will be assigning great weight to your testimony at the hearing.
Some other types of evidence that might be used are the forms that were filled out along the way, (e.g. letters that were written by friends of family or peers or coworkers, school records, etc. Those are what I call smaller bits and pieces of evidence, but the most critical parts are your medical records and your testimony at a hearing.