The PhD, or doctorate, began as a Doctorate of Law in 9th century Islam. In medieval Europe, the Doctorate of Law soon extended to a Doctorate of Philosophy. The term "philosophy" then included what we think of now as the separate fields of science, religion and the humanities.
Medieval universities generally had no fixed course curriculum. Instead, students would apply to a professor, generally a widely recognized authority, for permission to study under him. If he accepted the student's application, the professor would eventually require the student to demonstrate through extensive discussion his knowledge of the most important documents in the field.
At some point, the student would write his thesis--an original work of scholarship. Until the thesis had been written and accepted, the student remained a candidate for the PhD. After acceptance of the thesis and its successful defense, the student received a Doctorate of Philosophy. This essential candidacy structure remains in force today.
In modern parlance, the word "thesis," although it correctly applies to the PhD document, more often describes the document you write to obtain a master's degree. The word "dissertation" applies only to the original research document you write to obtain a PhD.
Once you have completed your postgraduate courses at a university, and often before, you will connect with a thesis adviser. Sometimes academic departments will assign you to a thesis adviser; often you will seek out a professor in the specific area where you intend to write your thesis, and ask him to become your thesis adviser.
At its best, the relationship between student and thesis adviser becomes an important collegial relationship. You will meet to discuss your proposed area of research or study. Your adviser will make recommendations and suggestions. Sometimes, for example, your adviser might suggest limiting your research area or might suggest modifying your approach. She will also know the academic preferences of your doctoral committee members and the approaches they will favor.
By the time you have completed your dissertation, you will also have taken a wide-ranging oral examination in your field: English literature, European history or some other general academic area of study well beyond the boundaries of your dissertation subject. The members of your orals committee will often, but not always, be the same faculty members who will review and approve your dissertation. Once you have passed your orals and have had your dissertation approved, you will receive your PhD.