STEAMWORKS is a digital distribution game service. This is their website. To download and run a steam game, you have to run the steam client or software. Everytime you run it it accesses the internet to validate your login and password. It can store your login and password for convenience. If it detects that there is no internet connection, it will offer to run in offline mode.
Steam has its own disadvantages. the steam client and some steam games can take many seconds to connect to the steam servers if something is wrong with your internet connection or router. Again it depends on your ISP if it is reliable or not. Sometimes I sometimes "hack" games to bypass the steam verification to be able to load up faster. Morally I believe that its OK since I paid for the game and I do not give or sell the game to other people, only I play it. Although the steam client has an offline mode, there is no way to force it to go to offline mode every time. The closest is to run it in offline mode then exit. The next time you run it it will ask you if you want offline or online mode. The steam client takes many seconds to shut down, and if its active when you shut down windows, it will add those many seconds to your windows shutdown time.
Special danger. In some cases, a combination of internet and/or router problems can fool steam into thinking that 1. there is an internet connection, therefore there is no reason to suggest going into offline mode and 2. steam cannot connect. I have not been able to replicate the conditions reliably but it has happened to me several times. If that happens, steam will delete the current password stored (even if you set steam to remember your password), assume that the current password is wrong and pop up an error message, asking you to input a new password and it will refuse to go into offline mode. Lets say your internet had say scheduled maintenance for say 1 week and it cut off in the middle of steam's logon sequence. If the timing is right, steam will, as explained above, delete your password and ask for a correct password. When that happens, since your net is down, you can't logon for 1 week. If you can't log on, you can't play your games until you log on. Workaround is a secondary internet connection like a 3g usb stick or a "hack", as mentioned above.