Understanding the Network Terms SSID
The terms BSSID, ESSID, and SSID are all used to describe sections of a wireless network (WLAN)—the three terms have slightly different meanings. As a wireless user you are concerned only with the broadcast SSIDs that let you connect to a wireless network. As an administrator, you also need to keep track of BSSIDs and, to a lesser degree, ESSIDs.
This topic describes:
- An SSID is the Name of a Network
- BSSIDs Identify Access Points and Their Clients
- An ESS Consists of BSSs
An SSID is the Name of a Network
Each BSS or ESS is identified by a service set identifier (SSID) - a 1 to 32 byte string. This is normally a human-readable string and thus commonly called the "network name". Because multiple WLANs can coexist in one airspace, each WLAN needs a unique name—this name is the service set ID (SSID) of the network. Your wireless device can see the SSIDs for all available networks—therefore, when you click a wireless icon, the SSIDs recognized by device are listed. For example, suppose your wireless list consists of three SSIDs named Student, Faculty, and Voice. This means that an administrator has created three WLAN Service profiles and, as part of each WLAN service profile, provided the SSID name Student, Faculty, or Voice. (For directions to create a WLAN Service profile, see Creating and Managing a WLAN Service Profile.)
As a WLAN user, you are concerned only with the SSIDs. You select one from the list on your laptop or other device, provide your username and a password, and use the SSID. You might not have access to all SSIDs—the authentication and access privileges are usually different for different WLANs and their associated SSIDs.
BSSIDs Identify Access Points and Their Clients
The basic service set (BSS) provides the basic building-block of an 802.11 wireless LAN. In infrastructure mode, a single access point (AP) together with all associated stations (STAs) is called a BSS; not to be confused with the coverage of an access point, known as the basic service area (BSA) 
Each BSS is uniquely identified by a basic service set identification (BSSID). For a BSS operating in infrastructure mode, the BSSID is the MAC address of the wireless access point(WAP) generated by combining the 24 bit Organization Unique Identifier (the manufacturer's identity) and the manufacturer's assigned 24-bit identifier for the radio chipset in the WAP. The BSSID is the formal name of the BSS and is always associated with only one BSS. The SSID is the informal (human) name of the BSS (just like a Windows Workgroup name). A BSS is functionally a contention domain as a local or workgroup network is functionally a broadcast domain.
Packets bound for devices within the WLAN need to go to the correct destination. The SSID keeps the packets within the correct WLAN, even when overlapping WLANs are present. However, there are usually multiple access points within each WLAN, and there has to be a way to identify those access points and their associated clients. This identifier is called a basic service set identifier (BSSID) and is included in all wireless packets.
Source: juniper.net and wikipedia.org
