The word Isomer comes from the Greek: Iso- same; -mer, unit. So isomers are molecules that have identical molecular formulas but have different structures (in one way or another.) For example, C4H10, can have 2 different structural arrangements (there's no way to draw them here, but essentially one would have a chain of 4 C atoms each bonded to the appropriate number of H atoms on each C (this is called butane); the other would have 3 C atoms in a chain with the 4th C atom coming off the 2nd C atom, again with the appropriate number of H atoms on each C (this is called 2-methylpropare or iso-butane). There are several other types of isomerism that you could look up. They include, but are not limited to, the following: geometric isomers, functional isomers, positional isomers, stereoisomers, optical isomers, conformational isomers, etc.
