Bija Mantras are the monosyllabic sounds. A bija mantra represents the essence of the mantra. Every devta has particular bija mantra. Generally a bija mantra consists of single syllable but some times it comprises several syllables also. There are five main bija mantras. The Bijas of the five elements, of Ether, Air, Fire, Water and Earth, are Haum, Yam, Ram, Vam and Lam respectively. The Sankrit word Bija in Hinduism and Buddhism is used as a metaphor for the origin or cause of things.
Some bija mantras are as follows:

Om
Om is most important among all the bija mantras. A majority of mantras begin and end with this sound. Om means yes and it affirms anything what is said after its pronunciation.

Haum
Here ha is Shiva and au is Sadashiva. This mantra is pronounced in the worshipping of Lord Shiva.

Hrim
It is a major mantra of all powerful goddesses and ruler of the world. This governs over the cosmic magnetic energy. Hrim destroys the worldly maya. According to the Vedas this is mantra of Sun. Hrim opens the lotus of the heart to the inner Sun of consciousness. It is a mantra of the heaven or the consciousness space in which all the worlds exist.

Krim
It is the mantra of great Goddess Kali. Kali is supposed to be the goddess of energy and transformation. This mantra grants all spiritual facilities and power. Krim has a special power relative to the lower chakras, which it can both stimulate and transform. Krim awakens and purifies the subtle body. As a mantra of work and transformation it is the mantra of Kriya Yoga, the Yoga of practice. It is the prime mantra of the Yoga Shakti. It should be used with care due to its strength.

Hum
Hum is a Shiva mantra but also a mantra of Chandi, the fierce form of Goddess Kali. It is used to destroy negative energy and creates great passion and vitality. As a powerful mantra it should also be handled carefully. Hum is a Vedic chant of fire. This mantra used to make offerings into the sacred fire.

Shrim
It is a mantra of love, devotion and beauty. This mantra relates Goddess Lakshmi, the Goddess of Beauty and divine grace. Shrim also relates to the head and can be used to flood the senses with divine beauty and delight. According to the Vedas shrim is a Soma mantra. It denotes love, joy, bliss, beauty and delight.

The bija mantras should not be chanted by the people who are not well acquainted by them as all the bija mantras are very strong and powerful.