This is one of the most common problems with laptops that comes with a touch-pad.
What usually happens is when you are typing, your palm touches the touch-pad which changes the cursor position and causes the text typed to be deleted, cursor change its postion or get messed up.
Most users are comfortable with disabling the touchpad and using an external mouse to counter this problem.
There are many ways to disable touchpad, most laptops comes with a function key which when pressed with the Fn key disables the Touchpad, and when repressed enables the Touchpad.
Have a close look at those keys to identify which one is with this icon. Once identified, hold the Fn key and then press the touchpad-f key.
On most Dell laptops, the combination is Fn + F5. Another method of tackling this problem is by using the touch-freeze program which disables the touchpad when typing. (recommended)
After the program has been download run it and follow the instructions on screen to finish the setup.