(* Standard levelspec for things like Map, Apply, Position, etc *)
(* n - levels 1 through n *)
Map[f,l,3]

(* Infinity - levels 1 through infinity *)
Map[f,l,Infinity]

(* {n} - level n only *)
Map[f,l,{3}]

(* {n1,n2} - levels n1 through n2 *)
Map[f,l,{2,5}]

(* The default levelspec is {1} *)
Map[f,l]===Map[f,l,{1}]

(* 
A positive level n consists of all parts of expr specified by n indices.
A negative level -n consists of all parts of expr with depth n.
Level 0 corresponds to the whole expression.
*)