Two molecules having the same molecular formula may be believed to look alike and related, but they aren't. The expanded structures of these molecules will reveal the truth - they differ in how the atoms are connected, making them structural isomers of each other. These variations are classified into six subtypes (chain, position, functional, tautomerism, metamerism, and ring-chain) and are covered in detail in the chapter 'Structural Isomerism' on curlyarrows.com.
CurlyArrows has created a beginner's guide- Introductory Organic Chemistry as a prerequisite to organic chemistry reactions. It comprises 13 carefully crafted, simplified, illustrative, and descriptive chapters.