What is isomers and isomerism by using butane as an example

Isomers are compounds that share the same formula but assemble the atoms in a different arrangement. Butane is C4H10, it has two isomers - butane and 2-methylpropane.
To draw them, draw 3 carbon atoms in a row. C-C-C. Do it again.
Now put a 4th carbon on either end of the chain, and put a carbon on the middle atom of the chain. Hook on the hydorgen atoms and you have your two isomers: n-Butane and isobutane.