The beacon frame is used by access points, and stations in an IBSS, to communicate their characteristics for connection. A beacon frame is sent periodically, called the target beacon transmission time (TBTT). Default time is 102,400 microseconds (102 ms). 100 time units (TUs) is 1,024 microseconds.
APs also contend for airtime while sending beacon frames. If the network is busy when an AP wants to send a beacon frame, then the AP will delay the beacon transmission.
Stations use access point beacons as a time reference for each beacon has a time stamp and an indication of when the next beacon will be sent. A station will use use the time stamp to make sure their clock uses the same tempo as the access point. This is called the timing synchronization function (TSF).
In an IBSS, the first station to create the ad hoc network defines the beacon interval and all stations joining the IBSS learn this interval.
To see an example beacon frame, download my pcap file captured from my laptop.
Links and Resources

* CTS 028: Management Frames
* Wireshark Display Filters via SemfioNetworks