A Beej Mantra is the shortest form of a Mantra just like a beej (seed) which when sown grows into a tree. Similarly beej mantras of different Gods, when recited together give humans lot of positive energy and blessings of all the Gods. They are the vibrations, and represent the "call" of the soul. It is belief that when the universe was created then the sounds produce during the cosmic evolution are basically the beej mantra.
Bija means a seed. Hence Bija Mantra means a mantra which is usually of one syllable. Accordingly the mantras which contain up to nine words are termed Beej Mantra, ten or twenty words forms Mantra and beyond are known Maha Mantra.
Basic beej mantra "Om" is further expanded into the following types of beej -- yog beej ,tejo beej, shanti beej and raksha beej, which are respectively known as aeng (aim) hreem, sreem, kreem, kleem, dum, gam, glaum, lam, yam, aam or um or ram.
The table below is not complete, but gives the most common bija mantras.