[ubuntu] How UBuntu installation process works - Does it download and install automatically?

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http://ubuntuforums.org – I just started migrating from Windows to Ubuntu 10.04 I have big question regarding installing new softwares. -- I want to install a software "ABC12" which is on the web (Let us assume it is Ubuntu version). What are the ways to install - I am very confused. 1. By typing - sudo apt-get install ABC12. What will happen - Will Ubuntu search in the whole WWW and - i) Download and - ii).Install ??? 2. What is the use of Ubuntu Software Center? Can I install the above ABC12 using this tool? 3. When I type 'sudo apt-get install SOME SOFTWARE NAME', I see message like "Could no (Hardware)