Collect

Collect[expr, x]: Expands expr and collect together terms having the same power of x.
Collect[expr, {$x_1$, $x_2$, ...}]: Expands expr and collect together terms having the same powers of $x_1$, $x_2$, ....
Collect[expr, {$x_1$, $x_2$, ...}, filter]: After collect the terms, applies filter to each coefficient.

- Collect[(x+y)^3, y]
- Collect[2 Sin[x z] (x+2 y^2 + Sin[y] x), y]
- Collect[3 x y+2 Sin[x z] (x+2 y^2 + x) + (x+y)^3, y]
- Collect[3 x y+2 Sin[x z] (x+2 y^2 + x) + (x+y)^3, {x,y}]
- Collect[3 x y+2 Sin[x z] (x+2 y^2 + x) + (x+y)^3, {x,y}, h]

→ Denominator