Photos

Tilaka is a mark worn on the forehead and other parts of the body (as seen on the photo left).
It is a decorative and is also an identifying mark. Worn by a priest, ascetic, or worshiper it shows which Hindu tradition he follows. It may be made with sandalwood paste, ashes (vibhuti), kumkum, sindhoor, clay, or another substance.
The pastes are applied to the forehead and in some cases to the upper part of the head.
Besides Chandan ball there are various stamps used to sanctify the body(as seen on the photo on your right).
This is done by deluding a Chandan with a little bit of water on the palm of one's hand then by dipping the stamp into it's fragrant paste same is applyed on the body (as seen on the photo).

The devotees of Vishnu apply tilak to purify themselves, as it is applyed with mantra, also it does reminds a devotee that his body is also residing place of Vishnu thus it is a sacred.

Various teological schools in India have different style and shape of their mark (Tilak), in here Tilak of Gaudiya Vaishnava school is presented.
If you wish to know more on the same subject please go to:
http://en.wikipedia.org/wiki/Tilak