Exchange server interface 2013

Client(In exchange): it's the interface B/W the users and their mailboxes.IN exchange client means in which interface you are accessing your emails. 

By using clients only mailbox enabled user can access resources from exchange server 

We have two clients for exchange 2013 
OWA(Free)Web browser 
MS Office outlook (paid) 

Disadvantages of OWA (from user point of view) 
1.No chance of downloading emails
2.No backup