Ganesh (sometimes spelled Ganesha, but usually pronounced "ga-nesh") is also known as Ganapati. He is the son of Parvati and Shiva. Lord Ganesha brings knowledge. But he is most often known as the "Breaker of Obstacles".
Ganesha is represented as a short, pot-bellied man with yellow skin, four arms and an elephant's head with only one tusk. In his four hands he customarily holds a shell, a chakra(discus), a mace and a water-lily. His unusual steed is a rat.
There are many versions of how he was conceived. The most popular version is that Shiva and Parvati were leading a quiet life on Mount Kailash, Shiva's abode. In fact, life was too quiet and sedate for Parvati, who had nothing much to do all day as her chosen lord was mostly either immersed in deep meditation or obfuscated by the fumes of ganja (marijuana). For company there was Nandi, Shiva's bull, and his host of ghosts, who are said to inhabit Kailash and give Shiva company, especially when he smokes ganja and wants some song and dance to liven things up a bit. Of course, Parvati was feeling bored and wanted someone she could be more involved with. So she began to call upon Vishnu in prayer to grant her a son. Vishnu soon granted her wish and Parvati gave birth to a beautiful boy child she named Ganesha. Read more here.