GatherBy

GatherBy[list, f] gathers elements into sublists that give the same value when f is applied.

GatherBy[list, {f1, f2, ...}] gathers into nested sublists using fi at level i.

Examples

Group by a function:

GatherBy[{1, 2, 3, 4, 5, 6}, EvenQ]
(* {{1, 3, 5}, {2, 4, 6}} *)

Group strings by length:

GatherBy[{"a", "ab", "abc", "bc", "c"}, StringLength]
(* {{"a", "c"}, {"ab", "bc"}, {"abc"}} *)

Please visit the official Wolfram Language Reference for more details.