Wireless roaming defined
When implementing a wireless network, it is possible to enable a feature called roaming. Roaming is what happens when a wireless client disconnects from one access point and connects to another as the client physically moves from the range of the first to the range of the second.
The term roaming however is used in different ways, depending on the context.
Strictly speaking, roaming involves multiple mechanisms that make such handoff from one AP to another seamless. That means that any communication taking place, such as a voice conversation, for example, will experience no discontinuity during the handoff. That requires specific mechanisms that can only be delivered using a WLC.
Note that moving from one AP to another is formally called a BSS transition. Seamlessly handing off a client in a BSS transition is known as a Fast BSS transition and is defined within the IEEE 802.11r standard.
However, roaming is sometimes used more loosely in a context where you have two access points that have the same SSID and are served by the same VLAN. In such a case, a wireless client will "roam" but may experience a momentary disruption in service. In such a case, it will retain IP addressing information as well as association with the same SSID.
There is a third case which may use the term roaming. When a client moves from one AP to another that uses a different SSID and VLAN. In such a case someone may call this roaming, but in truth it is not because it involves a complete disassociation from the first AP and a new association with the second AP.
In all cases, it is important to note that it is the client that decides if and when it will "roam" from one AP to another depending upon the signal strength received from each AP.