You will usually have several buttons known as a "power button" on a PC98 personal computer running Microsoft Windows NT 6.1 or later. Only one of them is an actual, real, power button that controls the power supplied to the computer. Some of them don't even have anything to do with the power to your computer at all. They are:
A switch physically incorporated into the power-supply unit, usually a two-position switch marked with a vertical line symbol (for "powered on") and a circle synbol (for "fully powered off").
A switch on the front panel, usually a sprung push-and-return button usually marked with a vertical line and circle arc (for "standby mode"). On a laptop rather than a desktop machine, this switch is usually positioned in a row of buttons above the keyboard, or on the side of the casing.
The "power button" on the Windows "Start" menu, in the lower right hand portion.
On older PC/AT and PC/XT equipment, pre-dating the PC97/PC98 standards, the switch on the front panel is a two-position, in and out, switch. It was wired into the power supply unit and interrupted the power supply circuit. One of the things that marked the transition from the PC/XT and PC/AT designs to the PC97/PC98 designs was that this switch moved to the back of the machine, becoming a two-position switch integral to the PSU itself, and a new switch on the front panel, that was a push-and-return button rather than a two-position switch, replaced it.
The two "power buttons" that actually affect system power are the one on the power supply unit and the one on the front panel. They are, in computer parlance, known as the "hard" and "soft" power buttons, respectively, as their distinguishing characteristics are supposedly that one operates entirely in hardware and the other operates entirely in software.
This is not really correct. The "soft" power button may be recognized in software, but may also be recognized in firmware; and indeed when the system is in standby mode it is actually recognized in hardware. It is better to recognize these buttons by their markings, which are actually standard electrical symbols:
The "hard" power button is marked with the standard electrical symbols (vertical line symbol and circle symbol) for full power off and power on. This button physically controls the power supply circuit. It renders the circuit open in the "off" position, and so can be validly marked with the circle symbol, which is reserved for switches that actually physically interrupt electrical power.
The "soft" power button is marked with the standard electrical symbol (vertical bar with a circle arc) for standby mode. This button does not physically interrupt any power circuit. Instead it sends a signal, that is recognized in software or hardware, causing the machine to take an action with some other, internal, mechanisms that physically power-up and power-down parts of the computer. (In standby mode, the circuit that powers the "soft" power button has itself to remain powered, of course, otherwise it wouldn't be able to send the signal requesting that the power come back on to the rest of the system. There is a system of separate power rails, for those parts of the system that are powered in standby mode and those that are not, on power supply unit connector cables.) This button is incapable of actually powering the entire machine off.
So remember: Unless you are pressing a physical button that is marked with the full circle symbol for full power off, you are not actually powering your machine entirely off. There's a reason that the so-called "soft power button" is marked with a standby symbol rather than a power off symbol, and a reason that good quality PSUs have a proper, circuit interrupting, power button, marked with the power off symbol, directly attached to them.
If you've ever configured the "power button action" in the Windows NT 6.1 "Start" menu to take an action such as "log off" and then pressed the power button only to see your computer not take that action, you've been mis-led by a particularly egregious piece of mis-naming in the Microsoft Windows Explorer shell.
The "power button" that it is referring to is not a physical button on your machine, but the graphical button in the lower right-hand portion of the start menu. The word "power" isn't being used here in the ordinary sense of electrical or motive power that drives a machine such as a personal computer, but in the sense of "power dressing". It's a button that can do one of a wide range of actions if pressed. Therefore, goes the reasoning, it is a "power" button. Those of us who think that the "power button" on a piece of electrical equipment is something to do with the electrical power to that equipment, be damned.