Do I need a ballast and what does it do?

been wanting to go with the hps lighting but i noticed they sell ballast next to the bulbs. so what does a ballast do? ive been told i can buy a up to 250 watt hps and stick it right in my light sockets and use em that way. Is this true? i dont want to start fires or blow my house off the face of the earth so i wanta know before i go and buy just a bulb or two.

yes u do...a normal light socket in ur home is only 120 volts.. hps lights can use more then that the ballast makes that all happen without ruing the buld... now what ever wattage ur bulb is get that ballast.... for best fit..... and if u have the extra money get the digital ballast its even better then the other one...hopes this helps