Collect

Collect[expr,x] collects together terms involving the same powers of objects matching x. 

Collect[expr,{x1,x2,…}] successively collects together terms that involve the same powers of objects matching x1, then x2, …. 

Collect[expr,var,h] applies h to the expression that forms the coefficient of each term obtained. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.