In Thailand the two main schools to which one can be ordained are Dhammayut and Mahanikaya. As we know maha means great so the Mahanikaya is the far larger school.
Dhammayut are mainly forest monks, more intent upon patipatta than pariyatta (practice than study). They often keep more strict rules such as only eating one meal per day, eating only from the alms bowl, not touching money, not receiving alms apart from the bindhabaht, not riding upon motorcycles etc.
They usually wear the dark brown robes.