To connect a cisco router to another cisco router

It depends
- You can directly connect two routers with cables or over the internet.
- The connection does not have to be serial although serial connections are widely used for WAN connection; RJ45 cables, Coaxial, or Optical connections may also be used.

Do the buildings have their onw internet connection already or are these routers going to provide internet access?

Thank you for responding, I understand if two office are distance apart you can use the internet to communicate using the routers. Company ABC firm started with 20 users and all of them were in one building, during the years it grew from 20 to 110 employees and now the firm is using two building distance less than 1,000 feet. They are using fiber optic to connect from one building to the other and each building has it own server one primary the other secondary. In case the primary fail (Hardware) the secondary take over using heartbeat technology.  

My question is:
1)What is the benefit having two sub net ( or more) ?
2)If you want to have more than one sub net and the router are far away between 150 to 900 feet, what type of interface you will use to connect the router?

Thank you again

I've requested that this question be deleted for the following reason:

I am still confused

no

Apologies if the above is not clear enough.
 Please look up video tutorials on subnetting on youtube.com

 Your question is what are the benefits of subnetting and the answer is

- Security eg restricting access globally based on subnet
  - Ease of management
  - Quality of Service (eg VOIP vlan should be dedicated and not shared with data etc. VOIP traffic should also be prioritized over other traffic)


 I hope that clears your confusion