Difference Between SSID, BSSID, and ESSID

ESS is the union of a set of BSS's. ESSID and BSSID are just their IDs respectively.

ESSID is the name of the access point, which can be changed. BSSID is a numeric ID of the access point (something like the MAC address of the router). For instance, on my college campus we have many different access points with the same name, but BSSIDs are different for each router.

Types of Service Sets: BSS (Basic Service Set) ESS (Extended Service Set). ESSs consist of one or more infrastructure-BBSs (the usual mode). They are associated with multiple access points. All the APs' beacons will broadcast the same SSID but different BSSIDs. It involves roaming. The user gets connected to the AP that has maximum strength. Usually, nearby BSSs broadcast on different channels/frequencies.

SSID: Network name (friendly, text, even with non-ISO basic Latin characters, up to 64 characters). Could be hidden (no broadcast). Sometimes "wrongly" called ESSID since it might group a set of APs under one name, but there is formally no such thing as an ESSID in the 802.11 standards. BSSID: MAC address of the access point, it uniquely identifies each one.