Welcome to Ubuntu !
Will answer you one by one, starting with simple ones !

• What is MD5
  
  This is an algorithm used to verify the consistency of your download.
  There are chances that during download the file may not get properly downloaded.
  
  Just to verify that the file now on your desktop is exactly the same as on the server, you calculate the its Hash (MD5).
  
  This is a alphanumeric string corresponding to a file, which is unique to it.
  If your MD5 string matches with the one on the publisher's website, then you have a proper download !
  
  
  




• Terminal in Minimal Install
  
  A minimal install of ubuntu is a bootable CD with file size of ~35MB
  
  This CD contains only the most essential files which merely boots up the system to and talk to your LAN port (RJ-45) with generic drivers.
  
  Once you are here, system requires active internet connection, and downloads freshest of the files and the end result is that you have a system with a potent terminal giving you complete control of your system.
  
  NOTE: This can't be done with any USB modem/Dongle !
  
  
  

Now to install the packages you need, you have 2 choices; use either of the utilities you wish to

• via aptitude
  
  This is almost a graphical installes, displaying you all the installed and available for installation packages. just expand the sub-categories and select all that you need!
  
  sudo aptitude in terminal will take you to aptitude menu !




• via apt-get (recommended)
  
  This is more powerful method and most of the ubuntuers use this..
  
  It is beyond the scope of this answer to explain all the possibilities of apt-get, and unlike aptitude it's not possible to play around apt-get as noob ! However this might help you kickstart !

NOTE: No matter whatever you use, in the end the installations are handled by [dpkg][4] which you might learn if really super interested & a geek !

• As a noob you may use a very powerful application "tasksel" which will help you install the packages (even a complete desktop env. or powerful LAMP server); and you don't have to bother about any of the packages dependencies.
  
  Moreover this has the power to auto configure itself according to the system you are using!
  LINUX is intelligent !
  
  This question points how to achieve tasksel and use it !




• Also, post launch of unity or GNOME3, I've started using "Cinnamon" Desktop environment
  
  and here is how to install it on minimal environment (even tough u'll have to fix icons and eye-candy because minimal system won't have then prior).
  
  You will also need a login manager if going for custom desktop environment !
  
  
  

• Installing firmware drivers
  
  Many major firmware drivers are not included in ubuntu by default just because they don't comply with Open Source license of ubuntu !
  
  However in almost every case it's extremely easy to get these drivers installed !
  
  Additional Drivers Utility may be installed from this ubuntu applications page !