Have dowloaded many apps. I don't know how to invoke the,execute or even find the. Can someone please help me? Joseph

Hi again Joseph,

Yep, when you log in you have started your Gnome Session (unless you choose otherwise at the login screen).

Applications which you download can either appear on the Applications menu under the various sub menus (Accessories, Games etc) or they may be programs designed to run from a command line - in which case they probably won't add a menu entry for themselves.
Sometimes the Applications menu doesn't update straight away and thus the programs don't appear there. The easiest fix for that is to restart your computer - there are other ways, but restarting is simple.

If a program doesn't appear on the menu, but you wish to run it, then typing the name of the program into a command line will start it and, if it is a GUI type program then the program will launch on your desktop (e.g typing firefox into the terminal and pressing enter will bring up the Firefox browser). There is a proviso that the application is located in your path, but don't worry too much about that - most things will work fine.
A non-GUI application (e.g the text editor Nano) will open in the terminal where you type the command -
so typing nano at the command prompt will bring up that program.

Starting programs from a command line is a good way to diagnose issues with stuff not starting correctly - you get feedback via the command line which will explain (although perhaps rather complicated!) what went wrong.

In general, applications should add a menu entry. If you find a specific app is failing to do so, then it may be worth mentioning it to the devs for that app, giving as much info as possible about your system, the version of Ubuntu, where you obtained the app etc.