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Briefly
In Exchange2010 high availability for the Client Access server was achieved through the configuration of a CASArray and some form of load balancing (hardware/virtual, or Windows NLB). Although the CAS Array no longer exists in Exchange2013, and other architectural changes mean that load balancing can be approached in different ways, the basic concept of a single namespace for Outlook connectivity remains
Find Two link below
From those2 links you can discover the difference
http://technet.microsoft.com/en-us/library/dd335211(v=exchg.141).aspx
http://technet.microsoft.com/en-us/library/dd638137(v=exchg.150).aspx
Regards
there are four key changes happened in2013:
Multiple databases per volume: This lets you have both active and passive databases on the same volume, which is great when you consider the larger disks that Exchange2013 can handle while still retaining low IOPS due to continued improvements to the information store and Extensible Storage Engine (ESE).◾Auto-reseed: If a disk fails and has one or more database copies on it, Exchange can reseed to another spare disk (if available) through the replication service. The service can remap a spare and reseed. If there's more than one database to reseed, Exchange can do them in parallel.◾Lagged copy enhancements: One change is the new Safety Net, which replaces the transport dumpster and provides a similar service of storing copies of messages successfully delivered to the active database of a mailbox server. (You can configure how long Safety Net holds these messages.) Safety Net is taking over the responsibilities of shadow redundancy in a DAG environment in that there is no need to keep another copy of a delivered message in a shadow queue while it waits to be replicated from the active to the passive database. That copy already exists in the Safety Net, so the shadow queue is redundant. One of the best features in Safety Net is the ability to activate a lagged copy: You can toss your transaction logs and mount the copy and let the Safety Net bring that database current.◾Site resilience: This is by far my favorite new feature. Many Exchange admins have complained that, in data center failovers, you have to manually intervene and perform a site switchover. Not only does Exchange lack automatic failover between sites that experience a loss of quorum due to WAN outages and so forth, its manual switchover process is frustrating to work through. Exchange2013 changes that: You can now have an automatic failover of your primary data center. There are some design conditions to take into consideration -- such as having three locations (two for Exchange servers and one for your witness server) -- but having the possibility of automatic failover is just awesome. If you do the manual switchover, Exchange2013 reduces the effort to three simple steps.
