Answer 1

eval is a function that will try to guess the type, the nature of it's content and throw the result.
For example if you type:
eval("3+2");
Output will be 5
and not "3+2"

Other example:
var test = 2;
eval("3+test");
Output will be 5

That sound magic but the function is slow to run and developpers kind of look down it, see it as dirty coding... only use when you have no others choices.