Atoms are constants that represent a name of some thing. The value of an atom is it's name. An atom name starts with a colon.
:atom # that's how we define an atom
An atom's name is unique. Two atoms with the same names always are equal.
iex(1)> a = :atom :atom iex(2)> b = :atom :atom iex(3)> a == b true iex(4)> a === b true
false, actually are atoms.
iex(1)> true == :true true iex(2)> true === :true true
Atoms are stored in special atoms table. It's very important to know that this table is not garbage-collected. So, if you want (or accidentally it is a fact) constantly create atoms - it is a bad idea.