Installing multiple packages at the same time

You can do this, why not!? You can run the following single command to install package-x and package-y:

sudo apt-get install package-x package-y

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Note:

It has been mentioned that you can add the -y flag to apt-get to answer Yes to all the prompts. This usually only saves you from having to say Yes once, anyways. It's nice to know about the -y flag, but be careful, because it can also automatically remove things that you may not want removed. Typically, I omit the -y flag and manually review all Added or Removed packages to prevent myself from making mistakes that could have horrible affects on my computer. However, apt-get upgrade -y seems to be a good option and less volatile.

sudo apt-get upgrade package-x package-y -y

To install packages use:

sudo apt-get install package-name

To install multiple packages simply list one after the other:

sudo apt-get install package-name1 package-name2 package-name3

To force apt-get install to answer yes to any are you sure you want to install this package? questions which may arise add a -y to the end

sudo apt-get install package1 package2 package3 -y

I'm not sure about other users but I find it helpful to keep a file that has a list of all packages/programs that I commonly use so that in the event I need to reinstall I can simply copy and paste the command rather than having to spend the next few weeks figuring out what I have and haven't got.

As Angel identified below keeping a copy of dpkg --get-selections will allow you to keep handy all the packages you had in case of reinstall. You can also restore pkg metadata by using apt-mark showauto > auto.lst and apt-mark showmanual > manual.lst to get metadata and apt-mark auto < auto.lst and apt-mark manual.lst to restore it.