Websphere Application Server Multiple Data Centers for High Availability

Active/Passive:
1)One data center is serving requests, the other is not performing work but is standing by in case it is needed. Application data and applicationstate are replicated asynchronously between data centers so that when an outage occurs, failover to the surviving data center can be accomplished quickly witha minimal outage (minimal could mean some number of minutes or even a few hours in this context).

2)It is the easiest and safest to implement since only one is serving requests -- but it is also the most expensive, at least in termsof hardware, since you have two data centers with 100 percent capacity

Active/Active:
1)Both data centers are serving requests. Because each data center is active, application data (minimally) and application state (possibly) must be shared between the data centers, and replication of the application data and application state needs to be synchronous, with a minimal latency, in order to avoiding consistencies.

2)It is slightly more complex than Active/Passive -- but with some additional effort, it can work in a reliable manner .

Reformed Active/Active:
1)The key difference in this variation to Active/Active is that the data centers do not share application state and, if possible, there is no sharing of application data -- or it is at least minimized, if it can't be avoided altogether. For this to work, you need to provide affinity for requests to a specific data center, which is best accomplished with a network switch that distributes the load to the data centers (which, naturally, means it doesn't reside in either data center).

2)is the most complex topology to implement; to do so requires a sizable investment in suitable network capacity between the data centers.
Source : http://www.ibm.com/developerworks/websphere/techjournal/0707_col_alcott/0707_col_alcott.html