Is Equestrianism related to riding on a horse back?
That is exactly what it is! Equestrianism is know as riding, horse riding, or horseback riding. This can even include vaulting with horses. 
What do you mean by the term vaulting ?
vaulting is gymnastics and dance on horseback. This can be done in competitions. Also in competitions is rodeo, horse racing and even polo.
 Is it an olympic event?
Yes Equestrianism made its Summer Olympics debut at the 1900 Summer Olympics in Paris, France.
Is it similar to hockey or polo sport?
Maybe not hockey but yes to polo. Polo is played on horseback.