Isomers are molecules with the same molecular formula but different chemical structure. Butan-2-ol and diethyl ether are isomers.
There are two principal types of isomers: estructural isomers and stereoisomers.
Structural isomers are compounds that have the same molecular formula but different conectivity, the atoms are arranged in different ways in both compounds.
Butane and 2-methylpropane are chain isomers.
2-Pentanol and 3-pentanol are position isomers.
2-Butanol and diethyl ether are functional group isomers.