Creating fields/instance variables

Is the normal way to create fields by creating a script and attaching it to an object within a prefab?

var field1; 
var field2; //both initialized when the obj is intantiated
var instID; 

It doesn't necessarily need to be in a prefab. You can just put it on an object in the scene. But yes.

Just make sure to specify your type, i.e.

var field1 : float; // or whatever