Yantra is a form used for worship/meditation. The word yantra means-'which controls & protects '(yam samyamanat trayate iti yantrah)'. It helps us to cotrol our mind and sensory organs which cause our limitations. These limitations are the main cause of all our problems. When we use a yantra, it helps us to experience our real nature.
Yantras are generally of geometrical forms. It doesn't mean that nothing else can be a yantra. Any form can be a yantra in special cases.
There are two types of yantra :
One which is used as a talisman - Dharana yantra &
the other used for worship/meditation - poojana yantra.
Every deity has a yantra.
Same yantra drawn with different colours can be used for different purposes.