What is the different between empirical, molecular and structural formulas (full and condensed format) and skeletal formulas?

Skeletal (or line) formula

Structural isomers are compounds with the same molecular formula but different arrangements of atoms.

Saturated compounds contain single bonds only and unsaturated compounds contain double or triple bonds.

Shortcut to drawing organic molecules

HONC rule

1, 2, 3, and 4 bonds for H, O, N, and C