Replication/log shipping to a standby site
1. Is there any data type or primary key dependency exists if I use log shipping?


Log shipping works on database log backups. Any DML operation is captured in logs, so it is applied to secondary. No question of datatype or primary key dependency for logshipping to work.


  2. Why I have to use network folders/directories. Why can I not use local folders to capture log files?


Logshipping uses copy, restore jobs. Copy job copies log backups from primary to secondary and restore job restores them. So, in this case, assuming log shipping is between different instances of SQL, how can one job have visibility if it is local path?


  3. What about the password between the two databases i.e primary and standby as primary will connect to standby?


Copy job of primary should have write access and restore job should have read access.


  4. Is there any method whereby I can open my standby database in read only mode and logs are applied without interrupting or disconnecting my users?


No, it's not possible in log shipping. AlwaysOn does this by providing read only replicas. Further quote from MSDN on two options for log shipping:

From Using Secondary Servers for Query Processing


  
    There are two options for configuration when you place the secondary database in standby mode:
    
    You can choose to have database users disconnected when transaction log backups are being restored. If you choose this option, users will be disconnected from the database each time the log shipping restore job attempts to restore a transaction log to the secondary database. Disconnection will happen on the schedule you set for the restore job.    
    
    You can choose not to disconnect users. In this case, the restore job cannot restore transaction log backups to the secondary database if there are users connected to that database. Transaction log backups will accumulate until there are no user connections to the database.
  





  5. I have a very large database i.e of 45 TB. So for disaster recovery solution what method do you suggest?


First you should have valid backups that are tested routinely. Then you need to ask yourself:


How much downtime can I afford?  
Budget my business can spend?  
Do I need secondary databases for reporting?


Based on above questions,you can see what are the total HA solutions available and what is best for me.


  6. Please also mention the difference between "Log shipping" &amp; "Always ON".


There are lot of differences you can you see below.

What is better to use - database mirroring or log shipping for SQL Server?