The term "PC" usually refers to a desktop computer that lacks a battery and is designed for use on a desk; the terms notebook and laptop typically refer to portable computers with integrated batteries and screens. The terms "notebook" and "laptop" are interchangeable.
Laptops and notebooks generally come as computers with integrated screens that fold up and down; by folding the screen on top of the keyboard, users can transport these devices in a backpack or small case. Desktops are larger and more modular. Most have ports for attaching input devices, such as keyboards and mice, and a port for attaching a monitor.
The terms "netbook" and "ultrabook" often refer to devices that are similar to laptops but smaller and lighter. There is no official definition of what qualifies as a netbook or ultrabook, and manufacturers have varying standards.
Laptops tend to be built in a highly integrated manner. Components may be soldered into place, which can make upgrades difficult or impossible. In addition, it's generally difficult to replace a screen, keyboard or cursor device, and the battery may be impossible to replace. One of the biggest differences between these two classes of devices is price. A laptop with the same specifications as a desktop typically costs significantly more.