Rom is a storage technology that allows to write the only 
once.After the data has written it can be read for a number 
of times wheras in ram we can  read and write the data any 
num of times.

ROM is a nonvolatile memory whereas RAM is a Volatile memory.

In general terms :

RAM = short term memory , ROM = Long term memory.

In Tecnical terms :

RAM = volatile type , ROM = non volatile type.

Usage:

RAM : used for storing frequently used data,applications
that are currently and frequently running are stored in
RAM.The dat a in RAM lost when power off.

ROM : For permanent storage.Data does't lost when the power
is off.

Reusage :

RAM : Data losts when power off,so easy reusable (volatile).
ROM : Can be reused by erasing or modifying the data by
electronically.

Why RAM instid of ROM :

 Data retrieving method is faster in RAM than ROM.But RAM
costs more.

RAM =RANDOM ACCESS MEMORY..IT IS VOLATILE MEMORY..MEMORY HAS
TO BE LOSE WHILE PC IS TURNED OFF.

ROM =READ ONLY MEMORY..IT IS NON VOLATILE MEMORY..ALL THE
BIOS INFORMATION STORED IN ROM..THIS DATA CANT BE LOSE WHILE
PC IS TURNED OFF..