Conductors- they are the substances which allow the current to flow through them easily. They have a large number of free electrons and they offer a very small resistance to the current flow. Two examples of conductors of electricity are impure water and mercury. Some others are copper, aluminium etc.
Insulators - they are the substances which do not allow current to flow through them. They have no electrons and offer a very high resistance to the current flow. Example - Cotton, rubber.