A touchpad is a Human Interface Device (HID) which provides data input to a computer. The data typically consists of spatial x-axis and y-axis coordinates representing the position of the user's finger on the surface. This position translates to a position of the cursor on a Visual Display Unit (VDU).
The technology employs a pair of capacitive sensing layers. When the pressure of the finger falls on the surface of the layers, it changes the separation gap between the two layers, which changes the capacitance value. This change in capacitance value translates to a relative position value.
These types of input devices can sense tap motion and drag motion and therefore ideal as a replacement for mouse input.
The touchpad is a very sensitive and delicate component that measures the pressure your finger makes when in contact with the surface. These can sometimes fail on a laptop; however, many laptops also have a serial and USB interface where a conventional mouse connects. Any operating system with plug and play capability will automatically detect the mouse and install the necessary drivers.
Laptops can operate fine with an external mouse. Older laptops have a PS/2, DB-9 port often marked as COM1. Many newer laptops will have a USB port, and it is possible to obtain a mouse that can connect to this port as well.
Typically secured to the underside on the top half of the plastic case, it is difficult to replace it due to the four screws that secure it.
The problem is in gaining access to this part of the laptop. However, it is not impossible and the best thing to do is to consult the service manual for your particular model of laptop.
Of course, when disassembling a laptop, always remember to remove the battery first.
With the metal plate removed, you can see that there is a single PCB and a ribbon cable that connects to the motherboard. The ribbon cable normally consists of connections for the left and right clicks switches, and the touchpad sensors. The ribbon cable is normally very reliable and does not break, and it is the sensor layers in human contact that breaks.
As you can see, the switches and the touchpad sensors are on the same PCB thereby saving space and manufacturing costs.
With the fast pace of laptop development, many manufacturers will not keep spare parts for a laptop that is over five years old. In this case, you will need to look to the second-hand market at places such as eBay where many sellers break laptops to sell their individual components.