Some products install internally in your computer while others are external boxes that pass data through the serial or parallel port.
External devices are much easier to install and generally provide better calling features.
Because they simply plug into a serial or parallel port on the back of the computer, you avoid any IRQ conflicts. Also, external devices tend to work more smoothly with Windows 95 Dial-Up Networking, and if you run into problems and have to reset the modem, it can be switched on and off without having to reboot your computer. Internal products, on the other hand, are faster. They don't have serial-port limitations and therefore can reach top speeds without any additional help. If analog phone usage is important to you, you're better off with an external device, because internal models don't allow you to process phone calls over ISDN unless your computer is running.