As in Lewis structures, covalent bonds are indicated by line segments, with a doubled or tripled line segment indicating double or triple bonding, respectively.  Likewise, skeletal formulae indicate formal charges associated with each atom (although lone pairs are usually optional, see below).  In fact, skeletal formulae can be thought of as abbreviated Lewis structures that observe the following simplifications:
*Carbon atoms are represented by the vertices (intersections or termini) of line segments.  For clarity, methyl groups are often explicitly written out as Me or CH, while (hetero)cumulene carbons are frequently represented by a heavy center dot. 
* Hydrogen atoms attached to carbon are implied.  An unlabeled vertex is understood to represent a carbon attached to the number of hydrogens required to satisfy the octet rule, while a vertex labeled with a formal charge and/or nonbonding electron(s) is understood to have the number of hydrogen atoms required to give the carbon atom these indicated properties.  Optionally, acetylenic and formyl hydrogens can be shown explicitly for the sake of clarity.
* Hydrogen atoms attached to a heteroatom are shown explicitly. The heteroatom and hydrogen atoms attached thereto are usually shown as a single group (e.g., OH, NH) without explicitly showing the hydrogen–heteroatom bond.  Heteroatoms with simple alkyl or aryl substituents, like methoxy (OMe) or dimethylamino (NMe), are sometimes shown in the same way, by analogy. 
* Lone pairs on carbene carbons must be indicated explicitly while lone pairs in other cases are optional and are shown only for emphasis.  In contrast, formal charges and unpaired electrons on main-group elements are always explicitly shown.
In the standard depiction of a molecule, the canonical form (resonance structure) with the greatest contribution is drawn.  However, the skeletal formula is understood to represent the "real molecule" that is, the weighted average of all contributing canonical forms.  Thus, in cases where two or more canonical forms contribute with equal weight (e.g., in benzene, or a carboxylate anion) and one of the canonical forms is selected arbitrarily, the skeletal formula is understood to depict the true structure, containing equivalent bonds of fractional order, even though the delocalized bonds are depicted as nonequivalent single and double bonds.