There are two service sets in IEEE 802.11 standard. The basic service set (BSS) for the infrastructure mode and the Independent Basic Service Set (IBSS) for the ad-hoc mode.
The BSS is a single access point (AP) together with all associated stations with the AP. In the infrastructure WLAN , communications can only take place between the AP and the other members of the BSS.
In the ad hoc networks, all of the stations those are within each other's transmission range is called an IBSS.