An atom in Clojure is a variable that can be changed throughout your program (namespace). Because most data types in Clojure are immutable (or unchangeable) - you can't change a number's value without redefining it - atoms are essential in Clojure programming.
CIDER is the acronym for Clojure(script) Interactive Development Environment that Rocks.
It is an extension to emacs. CIDER aims to provide an interactive development environment to the programmer. CIDER is built on top of nREPL, a networked REPL server and SLIME served as the principle inspiration for CIDER.