skeletal structure
Open any organic chemistry book and the molecules look like zig-zag lines and the occasional letter — almost no atom labels at all. This is the skeletal structure, the everyday shorthand chemists draw because it is fast and uncluttered. Every bend and every line-end is a carbon atom, and the hydrogens that fill out each carbon are simply not drawn. You see the molecule's shape and its functional groups instantly, without a forest of C's and H's.
The rules are simple once you internalize them. A line is a bond. Where two lines meet (a vertex) or where a line ends, there sits a carbon atom. Each carbon is assumed to carry enough hydrogens to reach four bonds total — this is the implicit-hydrogen convention. Atoms other than carbon and hydrogen (oxygen, nitrogen, chlorine, and so on) are always written out with their letter, and the hydrogens attached to those heteroatoms are shown too (the OH of an alcohol, the NH2 of an amine). So butane is just a zig-zag of three line-segments with four vertices/ends; ethanol is a short zig-zag ending in OH.
Skeletal drawings are the universal language of organic chemistry because they make structure and reactivity jump out: a ring is an obvious polygon, a double bond is a double line, a functional group is the one labelled spot your eye goes to. The cost is that beginners must train themselves to count the invisible carbons and hydrogens, and it is easy to forget a hydrogen or miscount a carbon when reading quickly.
A plain hexagon means cyclohexane (C6H12): six carbons at the corners, each implicitly carrying two hydrogens. Add a small circle or three alternating double lines inside and it becomes benzene (C6H6).
The same hexagon, hydrogens never drawn, becomes two different molecules depending on the bonds inside.
Carbons are invisible but never the heteroatoms: oxygen, nitrogen, halogens and the hydrogens bonded to them are always drawn explicitly. A line-end is a CH3, not a bare carbon.