Types of modem
There are 4 types of modem, which is Internal modem, External modem, On-board modem and Removable modem.
Internal modem are the modem that is NOT on a motherboard itself, but it is connected to a connector (Usually PCI slot). These modem can be called as network card. External modem are the modem between computer and the Internet. They usually contained in a box and connected using cables, usually serial or USB.
On-board modem is the modem located on motherboard itself. It cannot be removed but can be disabled when needed. Removable modem are wireless network cards connected through USB or older serial ports. It can be removed.
Versions of modem
Modem are classified by the rate of data transfer in a certain amount of time. They also classified by symbol rate (Baud) and connection method. Two main version are Digital Subscriber Line (DSL) and Dial-up.
DSL modem are very common to transfer data using the telephone line. There are a few types of DSL modem, but most of them are Asymmetric Digital Subscriber Line (ADSL). ADSL has different speed for uplink and downlink, that is, download speed is usually faster than upload speed.
An uncommon modem called Symmetric Digital Subscriber Line (SDSL) allow the speed to be similar on both download and upload. Very highspeed Digital Subscriber Line (VDSL) divides the telephony line and data line into two band, so telephony line won't interfere with data line, thus improving the speed.
Other than DSL, another type of modem called Dial-up is an older way of connecting to the Internet. User are required to use the dial the phone number of an Internet Service Provided (ISP) to connect to the Internet. This kind of connection method is not widely used but it is very common for the 90s as the speed is slower than other connection method.