Mobility applications are applications that handle transmission of data and user interaction between wireless devices and a central repository. Broadly, we can divide the mobile applications as synchronization, real-time, and hybrid applications.
Synchronization applications (SA) handle data transfers in situations where the user is not connected to the central database on a real-time basis. Instead of an instant data transfer, the user occasionally synchronizes the data residing on a mobile device with either a PC application or (potentially) a server. In this scenario, the user must schedule when the synchronization occurs and must be connected to a network of some type at that time. SAs work with a thick client that contains most or all of the data. During the synchronization process, the mobile device receives new or updated data, or the device sends its data to the PC or server. SAs are clearly very effective because they allow the user to access and carry critical information while on the go.  Synchronization in many cases should be the preferred means of data transmission, given the current wireless Internet networks' limited bandwidth and connection reliability, especially for business applications that are not time critical or that do require large amounts of data to be transferred back and forth from the device.
Real-time applications connect in real-time only. Examples include WAP connections over cell phones, PDAs with wireless Internet connectivity, or applications that are available if and only if there is a connection at the exact moment that the information is needed. The distinction here is that limited data is kept on the mobile device and the data that the user wants to see is accessible only at the time that a reliable network connection can be established. This type of application is the one we commonly think of when using the term mobile commerce, as the term implies that the activities are happening in real-time exclusively. Many applications that contain time-critical elements and low data volumes, such as financial trading applications, bidding in auctions, and status tracking, require such connectivity to function reliably and meaningfully.
Hybrid applications are only sporadically connected. These applications typically have a thick client, one that will process interactions in real-time if the real-time connection can be established, but queue the transaction and do something else (in the context of the business process) when the connection is not available. When the connection becomes available again, the queried interactions are processed at that time.
Many of the most-valued hybrid applications automatically detect the availability of a connection and then choose whichever means of connectivity is most applicable. The important part here is that the user of these applications does not have to know whether the applications are connected or not. The software performing the applications will take care of the connectivity and render it mostly invisible to the user. Obviously, if we are in need of real-time access to satisfy a query but are not connected, then we will receive an error message. However, most information can be fetched ahead; it can be synchronized, and it can be available with some degree of currency although the wireless connection may not be available at the exact time of processing.
Each application being designed today falls into one of the categories listed in this section. It depends on the situation and the real business needs that drive the application itself. If a user can satisfy his or her needs with an occasionally connected application, developing such an application is usually much less expensive and complex than building a real-time solution. Having the wireless Internet, real-time connectivity certainly has some benefits in terms of the data being current, but one most weigh the disadvantages of such an approach, mostly stemming from the fact that connectivity is not always available in key locations. Handheld devices come in all shapes and sizes, with many more currently being developed in manufacturers' research labs around the globe.
Cap Gemini Ernst & Young, Guide to Wireless Enterprise Application Architecture, John Wiley & Sons, New York, 2002.