Intellectual property(IP) is a term that is primarily used to refer to a combination of copyrights, patents and trademarks covering a certain work. It could also be used to refer to these three fields in general.
The primary idea of using the term "intellectual property" lies in comparing it to the "physical property" people actually own. Some use this to imply that IP should be protected in the same way as physical property (that is, eternally). In practice most or all copyright laws nowadays accept that copyrights expire a long time after the author's death.