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A wireless access point is a device that communicates with a computer's and devices via radio waves to provide network connectivity.
An SSID is the name of a wireless radio signal (e.g. HWS-Private). It identifies a particular set of network services offered from the access point.
A wireless computer can "roam" from one access point to another, depending on the strength of the signal coming from the access point. Your wireless network card will negotiate the best connection, and move from one access point to another transparently.