Anything we write or create that is original is our own intellectual property. Intellectual property is the legal term for the products of mind and creativity (writing and art) that are not primarily material in nature, and therefore can be copied easily. Intellectual property law seeks to protect the intellectual property of people and companies primarily through three tools: copyright, trademark, and patent.
If you write or create something while not working for someone else at the moment of creation, it is your intellectual property. If you wish to declare it as such, then you can mark it as copyrighted or trademarked. You can even apply for copyright or trademark, establishing legal proof that it is yours. And, if it is an invention, you can patent it.
Often, such steps are not needed. HubPages discourages copyright statements on HubPages as unprofessional.
If you are employed, then your work contract may mean that what you create on the job is a "work for hire" and belongs to your employer, and not to you.
Intellectual Property laws are different in different countries, and there are also international agreements. Intellectual Property is abbreviated IP, and this can be confusing, as IP also stands for Internet Protocol, a technical communications protocal underlying the Internet.