Generic power supply unit on computer question?

i have a ATX 500watts of PSU... they said it's generic?? what is even generic?? and its only cheap... and whenever i play games like deadspace my computer fastly overheats we can notice from the 1 fan that the heat coming out is very hot... and shuts down? im not sure whats the problem... the other fan at the back is geting weaker... im not sure if the problem is the psu or the other fan.. help pls