Chain isomerism
Chain isomerism arises due to the difference in arrangement of C atoms in the chain. For example, there are two isomers of butane, C4H10. In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is branched.
Position isomerism
It is due to the difference in position of functional groups.
For example, there are two structural isomers with the molecular formula C3H7Br. In one of them, the bromine atom is on the end of the chain, whereas in the other it is attached in the middle.