RAM stands for Random Access Memory
ROM stands for Read Only Memory
Computers need RAM because RAM is the place in the computers, where the operating system, application programs, and data (in current use) are stored that way the computers processor can easily reach it. ROM is memory that can not be changed by a program or the user. ROM has its memory even after it is turned off. ROM is an example of telling your computer how to start up.
Sorry that was kinda long. Hope this helps you out! Have an AWESOME rest of your day!