There are many types of devices that implement Ethernet like switches, hubs or routers. The early networks were using hubs to connect devices between each other. This implementation had some problems because all devices shared a common medium so collisions were frequent. This is why switches where invented, they are used to separate collision domains. Switches uses one port for each device and data is sent from one point to another directly without interfering with other devices.
MAC address is used to uniquely identify each device in a network. The source and destination MAC addresses are added when a packet is encapsulated into a frame. A MAC address is made of 48 bits composed of two parts. The first part (24 bits) identifies the company that produced the equipment and the last 24 bits are used to uniquely identify a device. Open a command prompt and type getmac to see the MAC addresses for the network devices attached to your computer (on a Windows OS):