As stated in other answers, usually when you install a package its executables are added to common paths, so if you know the name of the application you want to use, you just have to write its name.
If you want to know the command to execute a specific application this trick can help you:
xprop | grep WM_CLASS
this will let you click in any window you have currently open and probably get you the command needed to start that application. For example, clicking in a Document Viewer (pdf reader) window i get
WM_CLASS(STRING) = "evince", "Evince", being
evince the command used to open Document Viewer. If it don't works you can try replacing
WM_DESKTOP_FILE (this last will show the location of the .desktop file).
If you need to know which application provides the "command" you can use
But sometimes the program you are installing put their executables in some uncommon location. Take for example PostgreSQL. If you install any of the versions available in the repositories their executables are placed in
/usr/lib/postgresql/<version>/bin/. If you try to execute
initdb (a PostgreSQL command) the system will not be able to find the command.
In a situation like this you can use apt-file to search which package provides the
initdb program and where it will be placed after installing the package
apt-file search initdb
Also you can use
locate to try to find where the command you need is. Using again
initdb as an example:
Hope those tips help you.