Intellectual property is the capitalistic idea that "things" like ideas, knowledge and cultural works are the property of somebody - although they usually build on a whole tradition/frame of other ideas and knowledge. This concept leads to the idea that knowledge may be stolen by copying and sharing it - although it grows and gains from the sharing and nobody loses it.
There are several completely different forms of intellectual property. For this reason Richard Stallman encourages people to avoid the general term.
Copyright offers some opportunities for sharing: copyleft uses copyright law to ensure that intellectual works remain available for sharing.