@Joseph ****
Thanx, yep, that's a 90 day single entry Non-Immigrant Type O <- the O means "other". That visa is issued for a variety of reasons, being over 50, being married to a thai, being married to a foreigner legally working here, raising children here, volunteering, etc.
On the visa you have you were stamped in for 90 days when you arrived and you apply for an extension at the immigration office.