The two most basic types of communication are serial and parallel. They are so common that even the cabling bears the name serial cable and parallel cable. Since electricity behaves according to the laws of physics, it is impossible to get the electrical signal to go any faster. There are two ways to get the data from one place to the other faster. The first is to squish the data bits tighter together (leave less distance between them when they travel down the wire). The second way is to transmit more bits simultaneously.
Keep in mind that the information below is very general and not exactly correct from an engineering standpoint. We're just focusing on getting you to understand concepts here.
When information is sent across one wire, one data bit at a time, its called serial. Every computer on the face of the earth has some form of serial communications connector on it, whether internally or externally. Most people are familliar with the 'D' shaped 9-pin connector on the back of thier computer. This is a serial connector. The typical 9-pin 'D' shaped connector on the back of your computer uses 2 loops of wire (1 in each direction) for data communication, plus additional wires to control the flow of information. However, in any given direction, data is still flowing over a single wire.
Instead of squishing bits together, bits are sent over more wires simultaneously. In the case of an 25-pin parallel port, you have eight data-carrying wires so that eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being transferred eight times faster than a serial connection.