Variables/levels? Don't understand AS3

Hey I am just learning AS3 and trying to understand how variables and levels work now.
Before (in AS2) I would link to the variable in another MC by using root.mcname.variable but now that doesn’t work. And, for a variable I could use anywhere at any time in any MC, I would link to _level0.variablename but now that doesn’t work either. What is going on? How do variables work now (as well as levels) - sooo confused…this should be way simpler.