A thangka is a painting on cloth, flexible and because of that one can roll it. one of the meanings of the word f.e. is message.
Thangka's can be painted because of a lot of reasons : meditative, medicinal, astrological and as a means to convey some knowledge as stories of buddha's life or that of other great teachers.
They can also contain less religious items like lucky symbols.
A very impressive type of thangka's are the mandala's: symbolic circles which represent a mystic world or environment of deities.