Level

Level[exprlevelspec]
gives a list of all subexpressions of expr at the level(s) specified by levelspec.

Level uses standard level specifications:

n
levels 1 through n
Infinity
all levels from level 1
{n}
level n only
{mn}
levels m through n

Level 0 corresponds to the whole expression.

A negative level -n consists of parts with depth n.

Level -1 is the set of atoms in an expression:

Use the option Heads -> True to include heads: