1.Ruby complains if we try to evaluate an undefined local variable, but issues no such complaint for an instance variable; instead, instance variables are nil if not defined
2.Symbols look kind of like strings, but prefixed with a colon instead of surrounded by quotes. For example, :name is a symbol. You can think of symbols as basically strings without all the extra baggag
3.An array uses indices and a hash uses "keys"
4.This is not a standard rails directory, but a directory installed by RSpec in which all test files are kept.