Variables can be useful because they hold a certain number/data value that can easily be accessed and changed. For example, in a game, a players score is a variable that changes as the player gets more points; ie coins, items, etc.
Not a real memory but it points to physical memory.
A virus can do many things from burn up your hard drive to just annoy you with ads. the best thing you can do if you think you have one is take it to a professional